Schwartz and Dameshek1,2 have reported the apparent production of immunological tolerance in rabbits by the administration of a purine analogue, 6-mercaptopurine. Rabbits receiving concomitantly human serum albumin and 6-mercaptopurine failed to develop an immune response to the protein antigen, and weeks later, when antigen alone was administered, the animals remained tolerant. In an effort to confirm this finding and to explore the possible effects of 6-mercaptopurine on homografts in rabbits, two groups of rabbits received injections of ovalbumin in Freund's adjuvant and were grafted with skin from a common donor rabbit. One of these groups received 6-mercaptopurine in the maximum dose employed by Schwartz and Dameshek, the other group serving as controls. The 6-mercaptopurine did not produce immunological tolerance for ovalbumin nor did it result in successful homografts.