Both platelet membrane GP Ib and GP V have been proposed as receptors for the activation of human platelets by thrombin. Bernard-Soulier (B-S) platelets exhibit a reduced aggregation response to thrombin with a lag phase that precedes aggregation. When B-S platelets, whose surface proteins had been labelled with (125I), were analysed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) followed by PAS-staining, Coomassie blue staining or autoradiography, the apparent absence of GP Ib and the normal presence of GP IIb, IIIa and IIIb was demonstrated. On the basis of such studies several authors have stated that “GP I” is the thrombin receptor. However, GP V is not located by the above procedure, requiring more sensitive analytical methods for its detection. To meet this requirement washed platelets isolated from 3 B-S patients have been treated sequentially with neuraminidase, galactose oxidase and sodium(3H,)-boro- hydride. The labelled platelets were analysed by SDS-PAGE using 7-12% gradient acrylamide gels and the (3H,)-labelled GP’s located by fluorography. In addition to the GP Ib defect the platelets of each B-S patient were lacking the band corresponding to GP V of normal platelets. In agreement with previous studies we observed that when (3H,)-labelled normal human platelets were incubated with thrombin GP V (Mr=82,000) was hydrolysed,and that this was accompanied by the appearance of a labelled glycopeptide (Mr=69,500) in the supernatant. When (3H)-labelled B-S platelets were treated with thrombin no labelled glycopeptide was located. GP V could therefore be either absetit from B-S platelets or have a modified carbohydrate composition rendering it insensitive to the analytical procedure used. Interpretations into the reduced aggregation response of B-S platelets to thrombin should be extended to include a possible GP V defect.