Haemolytic assay for C8 revealed its association in functionally active form with washed human platelets. Platelet-bound C8 haemolytic activity was inhibited by F(ab')2 anti-C8 and was undetectable in the platelet suspension obtained from three C8 deficient patients. Incubation of platelets from C8 deficient individuals in normal plasma did not restore C8 haemolytic activity, indicating that platelets do not absorb C8 from plasma in vitro during platelet preparation. Thrombin, a mediator of the platelet release reaction, did not induce the release of C8 from normal platelets. Conversely, lysis of EAC1-7.9 by platelet bound C8 was not accompanied by release of beta-thromboglobulin or serotonin from the platelets. C8 was detected in a homogenate prepared from platelets as well as in the supernatant collected after high speed centrifugation of the homogenate. The association of C8 with platelets as an individual component rather than as part of the C5b-9 membrane-attack complex was supported by the following evidence: platelet bound C8 eluted from a Sephacryl S-200 column at the same volume as C8 from normal human serum; F(ab')2 anti-C8, but not F(ab')2 anti-C5, inhibited platelet C8 activity; the platelet homogenate, which lysed EAC1-7.9, had no effect on EAC43 which are susceptible to the lytic activity of the C5b-9 complex.