Blood lymphocytes from 53 patients with rheumatoid arthritis (RA) and 44 controls were cultured with the polyclonal B cell activator Epstein-Barr virus (EBV). Culture supernatants were removed at weekly intervals and the amount of IgM secreted by the lymphocytes measured by an enzyme-linked immunosorbent assay (ELISA). Three major differences in the pattern of EBV-induced IgM synthesis by RA versus control lymphocytes were observed. Lymphocytes from RA patients, in general, produced less IgM after one week in culture than controls. In contrast, they increased their IgM secretion significantly by the end of the second week, whereas control lymphocyte cultures showed little change in IgM secretion at this time. Control lymphocytes from EBV seropositive individuals produced undetectable amounts of IgM after five weeks in culture. However, lymphocytes from 40% of the RA patients, even though they were EBV seropositive, secreted greater than 2000 ng/ml (microgram/l) IgM after five weeks. The data are discussed in terms of defective B and T cell responses to EBV in lymphocytes from patients with RA.