A better understanding of the mechanisms involved in the proliferation of splenic colony-forming units (CFU-s) during tumor growth is important for the prevention of bone marrow aplasia during chemotherapy. The in vivo growth of EMT6 cells, a colony-stimulating factor-secreting mammary tumor, in BALB/c and nude mice resulted in splenomegaly and an increase in the number of splenic granulocyte/macrophage colony-forming cells (GM-CFC). Proliferation of CFU-s, observed in BALB/c mice but not in nude mice, most likely resulted from combined direct and indirect actions of factors secreted by tumor and host cells (in particular helper T cells). These factors were detectable in the serum immediately following tumor cell injection. Thus, the GM-CFC response to factors secreted by the EMT6 tumors is thymus-independent while the CFU-s response is dependent upon the presence of T cells. Finally, we show that EMT6 tumor growth had no effect on the determination of CFU-s differentiation toward the various myeloid cell lineages.