The content of macrophage colony-forming cells (M-CFC) and the serum colony-stimulating activity (CSA) were investigated in mice after intravenous administration of Lactobacillus casei YIT9018 (LC9018). In normal BALB/c mice, 500μg of LC9018 increased both femoral and splenic M-CFC; the highest levels were found a few days and a week, respectively, after the administration. LC9018 also induced an increase in splenic M-CFC in C3H/HeJ mice as well as in C3H/HeN mice, unlike lipopolysaccharide (LPS), which was ineffective in C3H/HeJ mice. In Meth A-bearing BALB/c mice, LC9018 (250μg×5) suppressed the growth of tumor cells and increased femoral and splenic M-CFC to much greater extents than Lactobacillus plantarum YIT0102 (250μg×5) did. LC9018 induced a rise of serum granulocyte-macrophage CSA in the same way as LPS. Sera taken 6hr after LPS administration, when transferred to normal mice, induced increases in femoral and splenic M-CFC. However, sera taken 6hr after LC9018 administration increased neither femoral nor splenic M-CFC. These results indicate that LC9018 modulates myelopoiesis at least at the stage of the proliferation of M-CFC in a different way from LPS, and this ability may be related to its antitumor activity.