Spontaneous in vivo proliferation of peripheral blood mononuclear cells (PBM) and T and B lymphocytes from 28 patients with rheumatoid arthritis (RA), 20 healthy individuals and 13 patients with psoriatic arthritis was evaluated. PBM, T (E+) and non-T (E-) cells from patients with active RA proliferated significantly more than the same populations from healthy individuals and patients with inactive RA. In contrast, only E+ cells from PSA patients showed a trend to increased proliferation relative to healthy individuals. AET treated sheep red blood cells (SRBC) inhibited 3HTdR incorporation of highly profilerating PBM of active RA patients. The data suggest in vivo activation of both B and T cells in patients with clinically active RA.