The interaction between fibronectin and C1q was studied in the presence of normal human plasma and rheumatoid synovial fluid by solid phase binding assay. Fibronectin-C1q binding occurred in the presence of rheumatoid synovial fluid but not in the presence of normal plasma. Binding was strongest at 4 degrees C and in the presence of EDTA. Fibronectin-C1q binding could be induced in the presence of normal plasma by hypotonicity, augmentation of the concentration of solution-phase fibronectin or by the addition of heat-aggregated IgG. The C1q present in rheumatoid synovial fluid bound to both aminoterminal collagen-binding and carboxyterminal noncollagen binding fibronectin fragments although binding to the aminoterminal fragment was stronger. The interaction between fibronectin and C1q in rheumatoid synovial fluid may modulate immune-complex deposition and complement activation in the inflamed joint.