Primary skin allografts taken from mice bearing Ehrlich ascites carcinoma and grafted to normal recipients underwent accelerated rejection, suggesting a quantitative change in the rejection process. Primary skin isografts from tumor-bearing donors also were observed to undergo rejection in 8–9 days when placed on normal recipients. Rejection of isografts suggests a qualitative change in the tissue of the tumor-bearing donor. Accelerated rejection of primary allografts by normal hosts apparently constitutes an expression of immunological memory in the absence of previous experience, i.e., no prior exposure to transplantation antigens.