The ability of the 125I-C1q binding test to detect soluble antigen-antibody complexes formed in vitro was tested with two antigen-antibody systems. Using tetanus toxoid:anti-toxoid complexes, all of the increased 125I-C1q binding activity was due to soluble immunoglobulin aggregates present in the unheated tetanus immune globulin. Using BSA:anti-BSA complexes, maximum 125I-C1q binding activity was present in the soluble supernatants from the equivalence zone. No immune complexes were present in these supernatants and the increased activity appeared to be due to alterations in endogenous C1 during the formation of large, insoluble immune complexes. The 125I-C1q binding test readily detected large, insoluble BSA:anti-BSA complexes but may not detect small, soluble complexes which have been suggested to be important in disease pathogenesis.