AbstractA heterogeneous group of proteins was separated from human serum IgG using 5 M guanidine hydrochloride solution in 0.1 M acetic acid. The protein mixture was fractionated by reverse‐phase high‐performance liquid chromatography and two proteins were isolated and identified as the C3a and C4a complement components (anaphylatoxins) according to their molecular masses and N‐terminal sequences. Using a chemical cross‐linking technique, the capacity of C3a and C4a to reassociate with the heavy and the light chains of IgG was shown. On the basis of the molecular masses of reconstituted complexes one molecule of C3a (or C4a) was bound to one heavy or one light chain of IgG.