A spontaneous B cell leukemia (BCL1) grew progressively in normal BALB/c mice after injection of tumor cells but did not grow in splenectomized recipients. Despite the absence of progressive tumor growth, residual tumor cells with malignant potential were found in the peripheral blood of the splenectomized animals. Splenectomy performed after injection of tumor cells but before the development of marked leukocytosis also prevented progressive tumor growth and death of the host. Thus the spleen appears to be necessary for progressive proliferation of this lymphocytic leukemia early after passage in vivo.