Forty three patients with systemic lupus erythematosus (SLE) were divided into three groups, inactive, active without treatment and active on treatment with steroids. T lymphocytes from peripheral blood were determined and the results compared with those of twenty five normal individuals. In further experiments, the mitogenic capacity of SLE lymphocytes to Con A and the serum's activity of these patients upon normal lymphocytes were analized. Decreased T lymphocytes were found in all groups of patients with SLE. There were no difference between the mitogenic response of SLE patients and controls when the lymphocytes were incubated with normal human AB serum. When normal lymphocytes were incubated with serum from SLE patients with active disease, and inhibition of mitogenic response to Con A was observed. These results suggests that the decrease cellular immunity observed in SLE is due to serum factors rather than to intrinsic T cell abnormality.