A simple and rapid procedure for the purification of C1q from human, guinea pig and mouse serum is described. This procedure allows the purification of C1q within one and a half days using euglobulin precipitation, chromatography on Superose 6B, followed by chromatography on Mono S by Fast Protein Liquid Chromatography (FPLC). The highly purified, hemolytically active C1q is free of any immunoglobulins. Since the purification of C1q of three different species was performed by the same purification procedure, a comparison of the subunit compositions was made under reducing and non-reducing conditions on SDS-PAGE. The yield was found to be more than 50%.