It is possible that a strategy designed to stimulate cancer cells in the active cell cycle may increase the effectiveness of S-phase specific anti-cancer agents such as methotrexate. In this study, the effects of granulocyte-colony stimulating factor (G-CSF) on the proliferation of cultured human bladder cancer cells and on the cytotoxicity of anti-cancer drugs to bladder cancer cells were studied in vitro. The 3H-thymidine uptake of cultured human bladder cancer cells, KU-1 and NBT-2, was significantly higher when the cells were treated with 10 ng/ml G-CSF than without G-CSF after 24- and 48-hour incubation. However, the cell numbers of KU-1 and NBT-2 were not significantly affected by 72-hour treatment with 10 ng/ml G-CSF. The binding of 125I-labeled KW-2228, a muteins of G-CSF, to KU-1 and NBT-2 was inhibited by unlabeled KW-2228 in a concentration dependent manner, which demonstrated the presence of G-CSF receptors on both cells. Scatchard analysis showed that the receptor densities of KU-1 and NBT-2 were 1770 and 3070 per cell, respectively. The combination treatment with methotrexate and G-CSF resulted in a significant increase in cytotoxic effects, when compared with methotrexate treatment alone. This study supports the possibility that the combination therapy of methotrexate and G-CSF increases clinical response in the treatment of advanced bladder cancer.