The accessory cell (AC) functions of peripheral blood dendritic cells (PDC) in concanavalin A (Con A)-induced T cell proliferation were investigated in patients with systemic lupus erythematosus (SLE). In Con A-induced T cell proliferation using added autologous PDC, AC activity was present in SLE patients (p less than 0.01). The Con A-induced T cell proliferation, however, was significantly lower in SLE patients than in normal subjects (p less than 0.01), and the level of decrease became greater with increased activity of the disease (p less than 0.01). In investigating the Con A-induced T cell proliferation using added allogeneic PDC, we compared T cells from SLE patients + PDC from normal subjects or T cells from normal subjects + PDC from SLE patients with T cells + allogeneic PDC both from normal subjects. The former two combinations showed significantly reduced T cell proliferation in active SLE (p less than 0.01 for each), while no significant differences were found for inactive SLE. The above results suggest that the reduced function of PDC as AC may be involved in reduced Con A-induced T cell proliferation in patients with active SLE.