A human colony-stimulating factor (CSF) producing cell line, T3M-1, has been established from explant cultures of a human squamous cell carcinoma of the oral cavity that secretes human CSF. It has been continously propagated during the past 15 months. The cells grew in a monolayered sheet with about 17 hr of population-doubling time and showed a colony-forming capacity with about 5% plating efficiency. The cells exhibited an epithelioid morphology resembling the structure of the original tumor, and they showed "tumor takes" when inoculated into nude mice. Karyotypic analysis revealed the cell line to be a human aneuploid one with a hypotriploid mode, including the Y-chromosome(s) and at least 10 common markers. T3M-1 cells possess the characteristic function of human CSF production in vitro, and a marked neutrophilia was observed in nude mice bearing the tumors produced by inoculation with the T3M-1 represents a new human cell line that secretes human CSF.