Lymphocyte responsiveness to mitogens (phytohaemagglutinin (PHA), concanavalin A (Con A), and pokeweed mitogen (PWM)), T-cell subsets (T mu and T gamma) and short-lived suppressor cell activity were investigated in the peripheral blood of seven patients with Behçet's syndrome and compared to normal individuals and patients with systemic lupus erythematosus (SLE). Amongst patients with Behçet's syndrome, responses to mitogens PHA and Con A were normal or slightly reduced; numbers of circulating T gamma cells were unaltered whereas T mu cells were reduced (P less than 0 . 05) compared with normal individuals. This was in contrast to SLE where a marked reduction in responses to PHA and Con A was found with reduced T gamma cell but normal T mu cell numbers. Although the mean suppressor cell activity in the Behçet's group was significantly reduced, all patients had values within the normal range, in contrast with SLE where the reduction was much more marked and most patients had values below the normal range. In conclusion, the pattern of alteration in T mu and T gamma cells in Behçet's syndrome is distinct from that in SLE, and the reduction of short-lived suppressor cell activity is only mild in Behçet's syndrome but marked in SLE.