The growth inhibitory effects of 5-fluorouracil (5-Fu) and the combination of 5-Fu and human leukocyte interferon (HuIFN-alpha) on the human salivary gland adenocarcinoma cell line HSG in culture were examined by measuring colony formation in the semi-solid agar medium and cell proliferation in the monolayer culture. As a consequence, the colony-forming ability of HSG cells in the agar medium containing 5-Fu was found to be markedly inhibited as compared with the untreated control. The concentration of 5-Fu yielding 50% inhibition of colony-forming ability of HSG cells under the presence of 5-Fu was 0.08 micrograms/ml. When the growth inhibitory effects of the combination of 5-Fu and HuIFN-alpha on HSG cells were examined, the colony forming ability of HSG cells was synergistically inhibited, whereas effects of the combined treatment on HSG cells in the monolayer culture were less sensitive than those on the colony formation. These findings strongly suggest that the combination of 5-Fu and HuIFN-alpha or 5-Fu alone is selectively effective on the neoplastic cells of HSG cell population but not on the non-neoplastic cells.