A new simple, rapid and economical method is described for producing monospecific antisera to human C1q without using gel filtration or column chromatography. Moderately purified C1q is obtained by dialyzing fresh human serum in the presence of chelating agents at low ionic strength and then electrophoresing it in agarose. When injected into rabbits, the electrophoretically purified product induced potent antisera to three or four serum proteins including C1q, all with slow electrophoretic mobilities (gamma to beta) at pH 8.6. The antibodies to serum proteins other than C1q are easily removed by immunoadsorbents consisting of the insolubilized supernatants obtained from the dialysis used to make the original C1q-rich fraction. The monospecific antisera prepared by this technique form only one band of precipitate in the slow gamma region in immunoelectrophoresis with whole human serum or C1q-rich solution, as well as in Ouchterlony double diffusion test. They agglutinate EAClq but not EA cells and detect the same antigen as standard monospecific antisera to human C1q obtained by another well-established method.