A simple, sensitive solid-phase radioimmunoassay to quantitate the activation of the classical pathway of complement by rheumatoid factor (RF) is described. RF (purified, in serum or synovial fluid) was bound to reduced and alkylated IgG adsorbed to polyvinyl chloride microtiter plates and reacted with diluted normal human serum (complement). The activation and binding of C4 were quantitated with 125I-Fab'2-anti-C4. Purified, polyclonal IgM--RF was 100- to 1,000-fold more effective than purified IgG--RF in activating complement. The amount of complement activation produced by RF in each of the 57 sera and 2 synovial fluid samples correlated directly with the amount of IgM--RF present. The complement activating abilities of polyclonal IgM--RF in the sera of 15 rheumatoid arthritis patients were homogeneous. This novel technique is readily applicable to the investigation of complement activation by RF in disease.