CD5 (OKT1, Leu-1) is an antigen originally associated only with T cells. This antigen has recently been detected on a population of B cells that have been implicated in autoimmune diseases, particularly in rheumatoid arthritis (RA). We determined the percentage of these cells in the peripheral blood (PB) and synovial fluid (SF) of patients with RA and other joint diseases (OJD) using flow cytometry and two-colour fluorescence. No significant difference was observed between the percentages of CD5-positive B cells in the PB of these two patient groups or healthy individuals. In comparison with PB, significantly higher percentages of these cells were observed in the SF of patients with RA, but not in SF of OJD patients. Higher percentages of B cells were also noted in RA SF. However, when B-cell percentages were accounted for, no significant difference was observed between the numbers of B cells expressing CD5 in SF. No correlation was observed between the percentages of CD5-positive B cells and the levels of rheumatoid factor (RF) or C-reactive protein (CRP). We conclude that CD5-positive B cells are an integral part of the B-cell pool of PB and that there is increased representation of B cells in SF. The increased percentages of B cells in RA SF, compared with OJD, may reflect the autoimmune phenomenon occurring in the rheumatoid joint.