Transplantation of a murine mammary carcinoma (CE maca) into mice induces marked granulocytosis and hypercalcemia secondary to excessive bone resorption. Such responses are not induced by another murine mammary carcinoma Bc66. In order to understand the mechanisms of these unique phenomena, we analyzed mRNA of tumor cells for expression of murine granulopoietic growth factors and studied interactions of tumor- derived factors using antiserum to a growth factor in vitro and in vivo. The Northern blot analysis of CE tumor clones revealed the expression of granulocyte colony stimulating factor (G-CSF) and macrophage colony stimulating factor (M-CSF), but no other CSF genes, while the Bc66 clone expressed only M-CSF. The G-CSF and M-CSF gene expression in CE tumor clones was accompanied by secretion of these proteins in culture. The granulocyte stimulating activity of CE tumor- derived G-CSF or recombinant human G-CSF was markedly enhanced by purified M-CSF in vitro. Significant but variable neutrophilia was observed in mice inoculated with CE tumor clones. Anti-M-CSF treatment of CE tumor-bearing mice significantly reduced neutrophilia, but did not affect hypercalcemia. These studies document that G-CSF and M-CSF are produced constitutively from the CE maca, and G-CSF is likely responsible for granulocytosis induced by this tumor. G-CSF and M-CSF function synergistically in granulocyte stimulation in vitro and this synergism may also play a role in marked granulocytosis of tumor- bearing animals, providing further evidence of the effect of CSFs in vivo.