In this report we have investigated the association of fibronectin with antigen-antibody-C1q complexes incubated in fibronectin-depleted and C1q-depleted plasma. When BSA--anti-BSA immune aggregates are incubated in plasma depleted of both fibronectin and C1q to which 125I-fibronectin has been reconstituted, little radio-activity is bound to the immune complexes. However, pre-incubation of immune complexes with purified C1q prior to incubation in the plasma causes an approximately 10-fold increase in the amount of radioactivity bound. The binding of 125I-fibronectin to preformed antigen-antibody-C1q complexes is specific, since the reaction is inhibited by the addition of unlabelled fibronectin but not by ovalbumin. When antigen-antibody-C1q complexes are incubated in C1q-depleted plasma containing physiological concentrations of fibronectin, and analysed by immunoblotting, fibronectin antigens are detected on the immune complexes. Identical results are obtained using immune complexes composed of sheep erythrocyte rabbit anti-sheep erythrocyte C1q (EAC1q) cells. There is no specific requirement for preformed antigen-antibody-C1q complexes, since fibronectin can be detected on antigen-antibody complexes after incubation in normal human serum or in C1q-depleted ethylenediamineteraacetic acid (EDTA) serum reconstituted with purified C1q prior to incubation with the complexes. Finally, we also demonstrate that in the presence of C1q, 125I-fibronectin will associate with soluble antigen-antibody complexes.