PBMC proliferation in patients with SLE was assessed by incorporation of 3H-Tdr in an accessory cell-dependent response to anti-CD2 specific monoclonal antibody. The response was compared to monoclonal anti-CD3 and PHA response. There was a marked decrease in the response to anti-CD2 in SLE patients (9257 +/- 8543) than in normal controls (20619 +/- 15279) (P < 0.005). It was more obvious in 8 patients with less active and untreated disease, but not in 10 patients with less active or inactive disease. In contrast, no statistical difference was noted in the response to anti-CD3 and PHA between SLE patients and normal controls. We also examined the response of purified T cells to anti-CD2 and the response was depressed in SLE, but no marked decrease in the response to anti-CD3 was found in SLE patients. Our results demonstrate that SLE patients with active disease have T cells that respond poorly to CD2 activation, but that response via the CD3/TCR complex is essentially intact. It might be reflect intrinsic T cell defects in some SLE patients.