While studying the unique Nramp1 (Slc11a1)-independent susceptibility to Mycobacterium bovis (BCG) infection of BXH-2 mice, we noted that these mice develop important splenomegaly and enlargement of lymph nodes. Segregation analyses in several F2 crosses showed that splenomegaly segregates as a single recessive trait caused by a novel mutation in BXH-2, independent of the infection. Histologic and fluorescenceactivated cell sorter (FACS) analyses indicated that splenomegaly is associated with a large increase in Mac1 ؉ /GR1 ؉ (macrophage antigen-1 ؉ /granulocyte differentiation antigen 1 ؉ ) granulocyte precursors in spleen, lymph nodes, and bone marrow, resembling a myeloproliferative syndrome. This is concomitant to extramedullary erythropoiesis in the spleen, as measured by proportion of Ter119 ؉ erythroid cells. The locus controlling this myeloproliferative syndrome and splenomegaly was designated Myls and maps to an 18 centimorgan (cM) region of chromosome 8, which also contains an integrated copy of an N-ecotropic murine leukemia virus (MuLV) provirus (Emv2). The relationship between Myls, expansion of Mac1 ؉ /GR1 ؉ cells, and Emv2 was investigated. Homozygosity at Myls is necessary but not sufficient for Becotropic virus replication in splenocytes, the extent of which appears to be under separate genetic control. Our results suggest a model in which Mylsdependent myeloproliferation in BXH-2 acts as a predisposing factor for the subsequent development of virally induced myeloid leukemia characteristic of this strain.