Treatment of rats with 25 mg/kg/day of cyclosporin A suppressed their immune response to skin allografts. Withdrawal of cyclosporin A treatment led to complete recovery of specific immune responsiveness and the time taken for recovery was independent of the duration of treatment. Titration of the dose of cyclosporin A administered in vivo indicated that doses of 12 mg/kg/day or less were not fully immunosuppressive.