C1q was purified to homogeneity from human plasma by a 3-step purification procedure. Plasma was euglobulin precipitated, and the redissolved precipitate chromatographed on a rabbit IgG-Sepharose column. The 1 M NaCl buffer eluate was passed directly through a rabbit anti-human IgG-Sepharose affinity column. C1q freed of IgG was present in the flow through. The rationale for this scheme to remove IgG free and that bound to C1q is discussed. Overall recovery of C1q was about 40% with IgG less than 4 microgram/mg C1q. In SDS-polyacrylamide electrophoresis with non-reducing conditions bands at 52,000 and 42,000 daltons were demonstrated while with reducing conditions bands at 26,000, 24,000 and 20,000 daltons were found as reported by others. C1q was found to be stable at 4 degrees C for 1 year in a 1 M NaCl, 0.4 M Tris, 10% sucrose, 0.005 M EDTA, 0.02% NaN3, pH 8.6 buffer.