Abstract The reversibility of interaction between rabbit C1q and a calcium dependent antigen-antibody system [GAT-anti-GAT-Ca+2], as well as reaction parameters thereof, have been carefully examined. By utilizing the reversibility of the GAT-anti-GAT-Ca+2 reaction by the addition of EDTA, it has been found that 1) C1q-IgG interaction is totally reversed by the disruption of the immune complex, independent of antigen-antibody ratios, 2) the amount of C1q bound to immune complexes mirrors antibody precipitated in the precipitin reaction, 3) the amount of C1q bound to “pre-formed” immune complexes is almost identical to that amount bound when immune complexes are formed in the presence of C1q, i.e., “dynamic complexes,” 4) C1q specifically bound to immune complexes can rebind as many as three times without loss of binding activity, and 5) C1q multiply reacted with immune complexes suffers no loss of hemolytic activity. Items 4 and 5 provide strong evidence that C1q remains native upon binding and that the labile nature of C1q is probably not related to its triggering actions in the complement phenomenon. From quantitative binding studies of C1q with pre-formed immune complexes formed at antibody excess, equivalence and antigen excess, the shapes of the binding curves and Scatchard plots are not suggestive of co-operative interactions between C1q and IgG. Association constants obtained with sheep or rabbit pre-formed immune complexes were significantly greater in antibody excess than at equivalence, as were amounts of C1q bound. For dynamic immune complexes formed in the presence of C1q, more C1q is bound at equivalence. Since more C1q was found to bind to these immune complexes at equivalence than in antibody excess, i.e., similar to an in vivo situation, it appears that quantities of C1q bound, rather than the energy of interaction, may be more related to the degree of complement fixation.