A method for determining immunological competence of mouse cells is described. This method is based on rejection of the erythropoietic component of homografted bone marrow by isologous cells. The extent of erythropoiesis was measured by spleen uptake of Fe59. On the basis of ability to reject homografted marrow, the following order of effectiveness was observed: leukocytes > lymph node cells > spleen cells > peritoneal fluid cells. Thymocytes and marrow cells showed no capacity for rejection. The immunological competence of cultured peripheral leukocytes declined rapidly so that at the end of 48 hr in vitro only 2.5% of the initial activity remained.