Monoclonal gammopathies in rheumatoid arthritis.

A total of 2560 sera from patients with and without rheumatoid diseases were examined for the incidence of monoclonal proteins. The incidence of monoclonal protein was higher (3.3%) in patients with rheumatoid arthritis than with other non-rheumatoid diseases of connective tissue (0.7%). Of 16 monoclonal proteins found in rheumatoid arthritis, 15 were IgG and 1 IgA; 7 sera belonged to the kappa and 9 to lambda type.