C1 and human platelets. II. Detection by immunological methods and role.

A C1 haemolytic activity was previously shown to be associated with platelets. The presence of C1 on human gel filtered platelets is confirmed by Ouchterlony analysis, and platelet aggregation induced by anti-c1 q antisera. After cleavage of C1 by EDTA, platelet aggregation induced by aggregated was decreased and was restored by the addition of C1. The possible role of C1 in human platelet aggregation induced by aggregated immunoglobulins or immune complexes is discussed.