AbstractPrevious work has shown that thymocytes of rats receiving two i.v. injections, 4 days apart, of large numbers of xenogeneic erythrocytes become tolerant of the injected cells whereas peripheral T cells in the same recipients make a vigorous response to the injected antigen. Results are herein presented of experiments on the effect of splenectomy on the induction of thymocyte tolerance produced in this way. Splenectomy performed either immediately before the first injection of erythrocytes, or between the two injections, completely prevented tolerance induction and tolerance was still incomplete when splenectomy was performed as late as a few hours after the second injection of tolerogen. Sham splenectomy had no effect on the erythrocyte‐induced thymocyte tolerance. The results demonstrate an essential role for the spleen in the tolerization of thymocytes in this system. Possible mechanisms and the implications for self tolerance are discusse