The present study was designed to evaluate the effects of a recombinant human granulocyte-colony stimulating factor (G-CSF) on leukopenia and tumor growth in mice treated with DA-125, an adri-amycin (ADM) derivative. In normal mice, single intravenous injection of DA-125 produced transient leukopenia accompanied with weight loss and splenic atrophy in a dose-related manner. However, subcutane-ous administration of G-CSF (5$mu$g/head) for 5 consecutive days after DA-125 resulted in a significantly elevated nadir of leukocyte counts and facilitation of recovery from the leukopenia. To investigate the effect of G-CSF on antitumor effects of DA-125, ADM (12 mg/kg) or DA-125 (40 mg/kg) was administered to Colon-26 murine adenocarcinoma-bearing Balb/c mice with G-CSF. Regardless of treatment with G-CSF, DA-125 and ADM markedly retarded the growth of implanted tumor, though they failed to increase mean survival time of tumor-bearing mice. These results suggest that G-CSF is able to not only ameliorate, but reconstitute DA-125-induced myelosuppression without affecting its antitumor potential.