To clarify the mechanism of inhibition of immune precipitation by early components of the classical pathway of complement, aggregation of 125I-BSA-rabbit-anti-BSA antibody complexes was performed in the presence of purified C1, C1-inactivator (C1-In) and C4. C1 delayed the rate of immune precipitation in a concentration dependent manner. This phenomenon was not influenced by the presence of 0.3 mM p-nitrophenyl-p-guanidinobenzoate (NPGB) which inhibits C1 activation. The antiaggregational effect of C1 was reversed by 10 mM EDTA and by C1-In at a C1-In/C1 molar ratio of greater than or equal to 4/1. C1-In was not effective when the reaction was performed in the presence of NPGB. Thus, although the inhibitory effect of C1 on immune precipitation was not dependent upon C1 activation, the formation of C1 was required to observe the effect of C1-In. The addition of C4 to C1 did not modify the slow aggregation of complexes, even when a limiting concentration of C1 was used. C1-In and EDTA were both able to cause similar rapid precipitation of complexes prepared in the presence of C1 and C4, demonstrating that C4 did not play a significant role in delaying the precipitation reaction. However, soluble complexes prepared in the presence of C1 and C4 were specifically precipitated by the addition of excess anti-C4 antibody, attesting to the binding of C4 to immune complexes. These observations suggest that the processing of immune complexes in vivo may not be similar in different classical pathway complement deficiency states.