A.

The background of the proceedings with which this opinion deals goes back to 1977. The plaintiff, AS-Autoteile Service GmbH, buys used motor-vehicle parts, in particular, clutches and clutch parts, reconditions them and sells them as replacement parts. In connection with its business it had dealings in spring 1977 with PAT, a company whose head office was in Meckenheim, near Bonn. The defendant, who is normally resident in France, was a shareholder in PAT. The plaintiff paid PAT a total of DM 1940949,95 for a number of deliveries, the last of which was on 9 September 1977. After the last delivery made by PAT differences of opinion arose over the quality of the goods supplied. The plaintiff brought a claim against PAT for the repayment of DM 1001476.95 on the ground that PAT had for the most part delivered only unusable scrap, instead of used parts suitable for reconditioning, as had been agreed. The plaintiff obtained default judgment for that amount in the Landgericht [Regional Court], Bonn on 5 April 1978. That judgment could not however be enforced, since PAT had become insolvent and an application for a winding-up order had been dismissed by order of the Amtsgericht [Local Court] Euskirchen of 5 May 1978 on the ground that there were insufficient assets to cover the cost of the proceedings. The plaintiff asserted that the defendant had received sums of money from PAT in a total amount in excess of its claim against PAT, and that he was obliged to repay them to that company under the law on unjust enrichment. On the application of the plaintiff PAT's alleged claims in unjust enrichment against the defendant were therefore attached and transferred to the plaintiff for recovery. The plaintiff thereupon brought an action against the defendant on those claims before the German courts in the amount of DM 1008741.25 plus interest. The action was dismissed as inadmissible, on the ground that the German courts lacked jurisdiction. In the judgment dismissing the claim in that action, the antecedent of the present proceedings, costs were awarded against the plaintiff. The defendant obtained an order against the plaintiff for the payment of those costs. To avoid enforcement of that order the plaintiff put forward a bank guarantee as security. Against the defendant's claim for the costs of the previous proceedings (about DM 40000) the plaintiff sought to set off the claims for unjust enrichment which had been attached and transferred to it for recovery and which it had unsuccessfully raised in those proceedings. In an application brought before the Landgericht [Regional Court] Baden-Baden, the plaintiff, relying on that right of setoff, claimed that the order to pay the costs of the previous proceedings should be declared unenforceable. The Landgericht dismissed the application as inadmissible on the ground that the German courts had no jurisdiction. It left open the question whether the German courts could derive jurisdiction from Article 16 (5) of the Brussels Convention. In its view there could be no jurisdiction over an application to oppose enforcement whereby the applicant, whose claim in previous proceedings was unsuccessful for lack of jurisdiction, sought to defeat the defendant's claim for costs arising out of those proceedings by setting off the claim in which he was unsuccessful. As permitted by Paragraph 566 a of the Code of Civil Procedure the plaintiff brought a direct appeal against that judgment before the Bundesgerichtshof. In order to arrive at a decision on the appeal the Bundesgerichtshof considered it necessary to obtain replies to three questions regarding Article 16 (5) of the Brussels Convention. It therefore referred the following three questions to the Court: (1) Do actions to oppose enforcement within the meaning of Paragraph 767 of the German Code of Civil Procedure fall within the jurisdiction provision contained in Article 16 (5) of the Convention; (2) Pursuant to Article 16 (5) of the Convention, in an action to oppose enforcement before the courts of the Contracting State in which enforcement is to take place, is it possible to resist enforcement by pleading a setoff in respect of a claim over which the courts of that State would have no jurisdiction if it were raised independently? (3) Does the jurisdiction recognized by Article 16 (5) of the Convention cover proceedings in which the debtor, relying on a submission that the creditor's claim cannot be enforced, demands the return of the instrument of guarantee which he provided as security to avoid enforcement? The plaintiff in the main proceedings, the Government of the United Kingdom and the Commission of the European Communities submitted observations regarding the reference. Those observations are in agreement with regard to the answers to be given to the first question and to the closely related third question. Both questions should, they say, be answered in the affirmative, since an action to oppose enforcement falls in principle within the scope of Article 16 (5) of the Brussels Convention. In view of the factual circumstances described above the question regarding the return of the instrument of guarantee should also be answered in the affirmative. There are differences of opinion, however, with regard to the second question, whether in an action to oppose enforcement claims may be pleaded over which the court hearing the matter, in the State in which enforcement is to take place, would have no jurisdiction under the general provisions of the Brussels Convention. The plaintiff in the main proceedings argues that the answer to that question should be in the affirmative, while the Government of the United Kingdom and the Commission are of the opposite view.