The intrathymic inoculation (ITI) of donor splenocytes into potential organ transplant recipients has been demonstrated to result in donor-specific unresponsiveness and greatly prolonged survival of subsequent organ allografts in rodents without the need for long-term pharmacological immunosuppressive therapy. We have studied the effect of the ITI of saline (controls) (groups 1 (n = 6) and 3 (n = 6)) or donor splenocytes (groups 2 (n = 10) and 4 (n = 8)) in dogs that received either pharmacological immunosuppression (with cyclosporine and prednisone, ±azathioprine/cyclophosphamide) (groups 1 and 2) or rabbit anti-dog antithymocyte globulin (groups 3 and 4) at the time of ITI. Kidney or heart allografting (from the donor of the splenocytes) was carried out 16-74 days after ITI; all but four transplants were performed within 16-22 days after ITI. Mean kidney allograft survival was 6, 10, 9, and 9 days, respectively, in groups 1-4. Mean cardiac allograft survival was 7, 14, 8, and 7 days, respectively. There was no statistical difference in allograft survival between those dogs that received ITI of saline and those that received donor splenocytes. These results would suggest that the protocols developed to date using ITI in rodent species may not be successful in dogs. © 1995.