The hemolysis of sheep red blood cells (SRBC) occurs via the classical complement pathway and is blocked by ethylene glycol bis-amino tetraacetate (EGTA). By contrast, fresh normal human serum in EGTA buffer was found to cause >90% hemolysis of unsensitized rabbit red blood cells (RaRBC) at a final dilution of 1:15. Absorbing human serum with RaRBC at 0°C will remove only 45% of this hemolytic activity and the same activity is present in human hypogammaglobulinemic serum. When rabbit lymphocytes were incubated with human serum in EGTA buffer, complement fixation occurred on their surface which was demonstrated with radiolabeled antibodies to human C3 or as “blocking” of the complement receptor. With purified complement components it was shown that the EGTA buffer completely blocked C1 but not C4, C2, or the late complement components. These findings show that a rabbit cell surface component can activate the alternate pathway of complement in human serum and suggest that this activation does not involve antibody.