Guinea pig γ1 immunoglobulins diminish the hemolytic activity in fresh serum when added as preformed immune precipitates. The loss in lytic potency is due primarily to interaction with the late C components beginning with C3. This fixation profile differs from that of the γ2 immunoglobulins which utilize all of the C components, whether they interact with antigen in the presence of C, or are incubated with C as preformed precipitates. Since the γ1 immunoglobulins do not cleave C3 in the presence of EDTA, it is suggested as one alternative, that a metal requiring enzyme other than the C4–C2 convertase may be involved in initiating C utilization by guinea pig γ1 immunoglobulins.