Inhibition of RTX-CDC is mediated by RF. A. Mixing studies using seronegative (RF-) and seropositive (RF+) sera demonstrates inhibition of CDC by RF+ sera, indicating that an inhibitor, not a factor deficiency, decreases CDC. B. Addition of monoclonal IgM RF markedly inhibits RTX-CDC by NHS. C. Addition of monoclonal IgA RF markedly inhibits RTX-CDC by NHS. Graphs represent average of three experiments; error bars represent mean standard error.