The three subcomponents of the first component of human complement, C1q, C1r and C1s have been separated and purified by a simple method with affinity chromatography on a column of chemically modified agarose to which human IgG has been covalently linked. This method employs human serum as a starting material, and separates active C1 from serum without the need for euglobulin precipitation. Large quantities of hemolytically active and physicochemically and immunochemically homogeneous C1s can be obtained by a subsequent preparative polyacrylamide electrophoresis step. C1q and C1r may be purified by DEAE-cellulose chromatography followed by gel filtration on Sephadex G-200. A fourth protein which was recovered from the affinity column appears to be a subcomponent of the C1 complex.