Human influenza A virus PR8 (Cambridge lineage) was grown in embryonated hens' eggs and harvested at a titre of 9610 8 pfu/ ml. For inoculation of the surfaces, the virus was diluted 1:10 in 1% BSA and serum free media (Dulbecco Modified Eagles Medium, DMEM, Gibco, UK). This represented a viral titre approximating 1.5610 8 TCID50/ml, just above the upper end of titres reported for human shedding [10, 11] . Preliminary experiments established that virus survival was improved by the addition of extra protein to the suspension. We tested 0.5% or 1% BSA as well as four preparations of artificial mucus produced from pig stomach mucosa (NBS Biologicals), pig stomach mucin types II or III or bovine sub maxillary glands mucin, type I-S (all from Sigma Aldrich). 1% BSA had the largest effect on titre and duration of survival, followed by the bovine mucin (data not shown). In the interests of simplicity and reproducibility, 1% BSA was therefore used in all subsequent experiments.