Pleura from 3 patients with rheumatoid pleuritis and from 5 patients with other diseases was examined for its capacity to synthesize IgM and IgM rheumatoid factor (RF). IgM was synthesized by pleura from all 3 patients with rheumatoid pleuritis and IgM RF by 2. Pleura from each of the control patients synthesized ≤1 ng/mg of IgM and IgM RF. Moreover, pleura from a 4th patient with rheumatoid pleuritis incorporated 3H-leucine into newly synthesized IgM. Simultaneously obtained pleural fluid mononuclear cells (MNL) and pleural tissue from a patient with rheumatoid pleuritis synthesized identical proportions of IgM and IgM RF whereas IgM RF production was not observed with peripheral blood MNL from the same patient. The results support the view that local production of IgM and IgM RF occurs in rheumatoid pleuritis and suggest that local immune events contribute to the pathogenesis of this disorder.