A minority of B cells express the CD5 marker, which is found on virtually all T cells, and CD72 has been defined as the CD5 ligand on the B cell membrane. The mean fluorescence intensity (MFI) of the CD5 molecules was shown to be higher on CD4+CD29+ than CD4+CD45RA+ in peripheral blood (PB) and synovial fluid (SF) of rheumatoid arthritis (RA) patients (P < 0.0001 and < 0.001), and PB of Sjögren's syndrome (SS) patients and normal controls (P < 0.02 and < 0.03). This MFI declined once the CD4 expressed HLA-DR in PB of SS patients (P < 0.004) and normal controls (P < 0.02) or CD25 in PB of RA (P < 0.004) and SS patients (P < 0.0004). There was a correlation between the CD5 MFI on CD4+CD45RA+ and CD4+CD29+ in RA (P < 0.001) as well as SS (P < 0.0007) PB. The CD72 MFI was impressively higher on CD5+ than CD5- B cells in PB and SF of RA patients (P < 0.0001 and P < 0.005) and PB of SS patients (P < 0.005) and normal controls (P < 0.005). Our data suggest that, in association with CD4CD29, CD5 is involved in CD5+B/CD5+ B cell interactions in non-organ-specific autoimmune diseases.