Methods for preparing chicken erythrocytes (CE) with C7 bound to their surface were devised by using either the classical pathway (CEA1-7) or an activated 56hu reagent (CE567) derived from inulin-treated human serum. These intermediates were used to study the lysis of CE by functionally purified C8 and C9 isolated from guinea pig and human serum. The results indicated that GPC9 was less efficient in lysing CEA1-8 or CE5678 than HuC9. This finding was observed irrespective of the species of C8 used. Experiments designed to analyze this difference indicated that there were two functionally distinct forms of C5b-8 that were randomly distributed among the cells but differed in their ability to generate a C lesion depending on the species of C9 used to complete the reaction. The implications of these results on the mechanism of generation of C lesions are briefly discussed.