The immune response was assessed in 13 competitive bodybuilders self-administering anabolic-androgenic steroids and ten competitive bodybuilders not administering these drugs. Laboratory assessment included the number and relative distribution of T-cells, T-helper/inducer cells, T-cytotoxic/suppressor cells, activated T-cells, lymphocyte transformation to the mitogens, pokeweed mitogen (PWM), phytohemagglutinin (PHA), Concanavalin-A (CON-A),Staphylococcus aureusCowan strain I (SAC), serum immunoglobulins, and natural killer (NK) activity. There were no significant differences in T-cell subsets among steroid users and non-users, but lymphocyte transformation studies revealed that the anabolic-androgenic steroid-using group had enhanced proliferative ability to the B-cell mitogen, SAC, in comparison to non-bodybuilding controls. NK activity was significantly (P< 0.05) augmented in the anabolic-androgenic steroid users but not in the non-using bodybuilders. Serum immunoglobulin levels, in particular IgA, were significantly (P< 0.017) lower in the steroid-using group. Four of 13 steroid users and three of eight non-steroid-using bodybuilders had detectable antinuclear antibodies. These studies indicate that 1) anabolic-androgenic steroid use as practiced by contemporary athletes is a potent modulator of immune responsiveness and 2) autoantibodies are prevalent in strength-trained men even in the absence of anabolic steroid use.