Prednisolone and antithymocyte globulin (ATG) were compared for their abilities to reverse acute rejection in cyclosporin A-treated canine graft recipients. Outbred dogs bearing kidney allografts were treated with a suboptimal dose of cyclosporin A (10 mg kg-1); at the onset of an acute rejection episode, animals received daily prednisolone (40 mg kg-1) or ATG (20 mg kg-1) until serum creatinine levels decreased. In seven untreated allograft recipients, rejection was first diagnosed at a mean 4.4 days post-transplant. In 23 dogs receiving cyclosporin A, rejection was first diagnosed in all dogs at a mean 43.6 days post-transplant. Compared to ATG, prednisolone was more successful in the reversal of primary acute rejection episodes (5/8 and 7/8 reversals, respectively) and produced a quicker return to normal renal function (mean 15.4 and 6.8 days, respectively). ATG therapy, however, resulted in fewer subsequent acute rejection episodes than prednisolone therapy (mean 1/5 and 6/7 developed secondary rejection episodes, respectively); the possible advantage of 'clonal depletion' following ATG therapy is discussed.