Macrophage colony-stimulating factor (MCSF) was used in the treatment of pancytopenia for a 64-year old man with prostatic cancer with bone metastases. Pancytopenia was improved and tumor markers (PA, PAP, gamma-Sm) were normalized rapidly after castration. The alkali phosphatase (A1P) concentration decreased to the normal range and bone metastases were improved markedly after two courses of chemotherapy. To determine the clinical efficacy of M-CSF against tumor markers and A1P, we used M-CSF on two other patients with prostatic cancer with bone metastases. In one of them, the levels of the tumor markers decreased from the initiation of the injection of M-CSF, but gradually increased to the initial levels after the last injection. The total A1P concentration was not changed by M-CSF, but the A1P-3 concentration decreased and continued to decline. These findings suggest that M-CSF has an antitumor effect and that it can be effective in the treatment of prostatic cancer with bone metastases.