Ten infertile men with significant titres of antisperm antibodies were entered into a randomized double-blind study to test the efficacy of corticosteroid therapy compared to a placebo. Prednisolone was given orally at a dose of 1 mg/kg/day for 9 days repeated over 3 cycles (their wives' menstrual cycles). This treatment had no significant effect, when compared to placebo, on fertility, serum antibody levels or semen characteristics. A slight decrease in the titre of seminal antibodies was, however, observed in patients receiving prednisolone. In individuals, there were important fluctuations in antibody levels which were independent of drug administration.