Collagens I and III, in fibrillar form, bound platelets equally well; both readily induced platelet aggregation. In contrast, collagens IV and V, although pretreated as collagens I and III to induce fibrillogenesis, failed to produce aggregation. No binding of platelets was detected. Lens capsule, containing collagen type IV in situ, was also inactive. Inactivity appears due to the lack of an appropriate quaternary structure since segment-long-spacing (SLS) aggregates of collagens IV and V, as of type I, induced aggregation. Elastin and its associated microfibrillar element did not aggregate platelets; some binding of platelets to elastin only was observed.