Purified preparations of WT virus, or of virions lacking individual glycoproteins, were produced as previously described [20] . Purified preparations were assayed for particle numbers using electron microscopy [21] and for infectivity by plaque assay on Vero cells or, in the case of glycoprotein-deficient virions, on the relevant helper cell line. Preparations of WT virions had a particle to infectivity ratio of 3610 1 . Preparations of gHnegative, gD-negative and gB negative virions had particle to infectivity ratios of 2610 10 , 5610 6 and 3610 6 respectively. The maximum virion dose of entry-incompetent virions used in the experiments described in this paper was 1000 particles per cell and it therefore follows that in the maximum infectivity of a preparation of an entry-defective virus (i.e. using gB-negative virions) would be less than one cell per thousand. Virus preparations were deglycosylated with 500 U of PNGase F for 18 hrs at 37uC in the absence of denaturing buffer [22] . Removal of N-linked sugars was confirmed by a shift in the electrophoretic mobility of gD on a protein denaturing gel.