Summary: Active C′1s was isolated from the euglobulin fraction of human serum by column chromatography on CM and DEAE cellulose and Pevikon block electrophoresis. The final product revealed a single precipitin line against horse anti-whole human serum on immunoelectrophoresis and did not contain C′1q. A spectrophotometric assay was introduced for measuring the esterase activity of C′1s using TAMe as a substrate. Active C′1s was capable of generating SAC′4,2a although it did not bind to EAC′4 cells and EA. The esterase activity and the hemolytic activity as well as the C′2 and C′4 destructive activity of active C′1s were associated with a single homogeneous protein which migrated to the α-globulin region and had a molecular weight of approximately 110,000. A molecular titration of active C′1s is not possible because active C′1s transfers from site to site; however, SAC′4,2a generating activity could be correlated with TAMe esterase activity. As the ratio of hemolytic to esterase activity of active C′1s was 20 to 40 times less than that of C′1a, the firm binding of C′1a mediated through the combining site for antibody, presumably on the C′1q subunit, increases the efficiency of cleavage and transfer of C′2 at SAC′4.