This laboratory and others have previously shown that the intrathymic injection of donor cells or major histocompatibility complex allopeptides induced indefinite survival of a subsequent graft without immunosuppression. This approach may open interesting new perspectives for transplant medicine. Studies to explore the feasibility of the technique in humans can only be designed with some form of concomitant immunosuppression to avoid the risk of irreversible rejection in the case that the thymus approach fails. Thus, one of the first issue to address is whether conventional immunosuppression interfered with the process of thymus tolerance. This study was designed to investigate the above issue. In transplanted Lewis control rats, cyclosporin A (CsA) (10 mg/kg per day) and methylprednisolone (MP) (10 mg/kg twice daily) for 3 days were invariably followed by kidney graft rejection within 10 days. In subsequent experiments, five groups of Lewis rats were injected with medium alone or Brown-Norway (BN) leukocytes into the thymus, and 24 h later, they were orthotopically transplanted with major histocompatibility complex-incompatible kidneys from BN rats. At the time of transplantation, Lewis rats received MP (10 mg/kg twice daily) CsA (10 mg/kg per day), or the combination of the two (MP+CsA at the same dose) for 3 days. Lewis rats injected intrathymically with BN leukocytes but not receiving immunosuppressants had indefinite survival of their kidney graft. The effect of the intrathymic injection of donor cells of inducing unresponsiveness to a subsequent kidney graft was abolished by concomitant immunosuppression. All animals given immunosuppressants rejected their graft within 12 days after surgery.(ABSTRACT TRUNCATED AT 250 WORDS)