The aggregation of platelets induced by collagens is considered an important step in primary hemostasis. Glycoprotein (GP) IIIb (GPIIIb, GPIV, CD36) has been proposed as a blood platelet receptor for collagen. Platelets from three healthy blood donors were shown to be clearly deficient in GPIIIb. These platelets aggregated normally in response to type I and III collagens. In addition, platelet factor 4, beta-thromboglobulin, and adenosine triphosphate (ATP) secretion in response to type I and III collagens was normal. The findings indicate that GPIIIb is not the major, essential collagen receptor for type I and III collagens. This would explain why all individuals with GPIIIb-deficient platelets examined so far are healthy and, in particular, show no apparent evidence of hemostatic problems. However, in contrast to control platelets, no aggregation and impaired platelet factor 4, beta-thromboglobulin, and ATP secretion was observed in response to type V collagen. Therefore, it is postulated that for type V collagen-induced aggregation both GPIa/IIa and GPIIIb are essential.