Mouse serum and EDTA plasma were subjected to low ionicity precipitation, ion exchange chromatography and gel filtration in an attempt to separate and purify C1, C4 and C2. Although they could be obtained free of many other serum proteins these three components could not be significantly separated from one another using a wide variety of ion exchange resins and gels and conditions of elution. C1, C4 and C2 eluted from Sephadex G-200 in buffers containing EDTA with an apparent molecular weight of approximately 150,000 and in buffers containing Ca++ and Mg++ with an apparent molecular weight of over 500,000. Cell bound activity can be formed by incubation of EA in mouse serum as indicated by the lysis of these cells upon the later addition of guinea pig serum in EDTA. Incubation of EA in mouse serum followed by incubation in buffers containing 10 mM EDTA resulted in a rapid loss of all detectable C1, C4 and C2 functional activities.