Unstimulated and pokeweed mitogen (PWM)-stimulated "in vitro" IgM synthesis by peripheral blood lymphocytes has been evaluated by an ELISA method in 33 patients with juvenile rheumatoid arthritis (JRA). Lymphocyte PWM responses were significantly reduced in patients with JRA compared with healthy controls. When patients were divided according to disease activity, a significant increase of stimulation index (ratio of IgM synthesis with over that without PWM) has been observed in patients with disease remission compared with those with active disease. These findings confirm the defect in PWM-induced IgM synthesis by peripheral blood lymphocytes of patients with JRA and suggest that these abnormalities may be related to disease activity.