The presence of factors implicated in B cell proliferation and differentiation was studied in synovial fluid (SF) from patients with rheumatoid arthritis (RA) and from patients with ankylosing spondylitis (AS) and traumatic joint injury. Culture with Staphylococcus aureus Cowan I B cell blasts showed strong B cell growth factor (BCGF) activity in the SF from patients with RA. This BCGF activity was significantly greater than that found in SF from patients with traumatic joint injury and similar to that of patients with AS. The presence of B cell differentiation factor (BCDF) for IgM(mu) in the SF from patients with RA was also demonstrated and was significantly greater than that found in SF from patients with AS and traumatic joint injury. Moreover, a significantly increased BCDF for IgG(gamma) was also found in the SF from patients with RA compared with that observed in those patients with traumatic joint injury, which, however, was similar to that of patients with AS.