Effects of human recombinant granulocyte colony-stimulating factor (G-CSF) on peripheral white blood cell (WBC) and platelet (PLT) counts were examined in BALB/c mice pretreated with various kinds of anticancer drugs. Significant increase of WBC and significant decrease or tendency to decrease of PLT count were observed in mice treated with ACNU (40 mg/kg), CY (150 mg/kg), MMC (8 mg/kg) or VDS (3 mg/kg) followed by G-CSF treatment. These results indicate that we should study more about a possibility that treatment with G-CSF and anticancer drugs may cause in man a more severe thrombocytopenia than is observed with only chemotherapy treatment.