A new tissue culture line, OAT-1975, has been established from explant cultures of human oat cell carcinoma of the lung, characterized and maintained for more than 1 year. The cells grew in a monolayered sheet with 24 hr of population-doubling time and are capable of clonal growth with about 60% plating efficiency. The cells still retain unique morphological and structural characteristics of oat cell carcinoma and showed "tumor takes" when transplanted into nude mice and conditioned young Syrian Golden hamsters. The chromosome constitution of the cells is hypertriploid with a modal peak at 74. The permanent growth integrity of the present cell line made it possible to utilize it for cell biological studies. Cell inactivation experiments indicated that the cells are sensitive to X-rays and Mitomycin C when compared to the responses of HeLa S3 cells.