AbstractAn interleukin‐2 (IL‐2) dependent murine cell line (TN‐9) which could be grown continuously with the crude culture supernatant of concanavaline A‐stimulated rat or mouse spleen cells could not synthesize DNA in the culture medium supplemented with partially purified or recombinant IL‐2. The cell growth was restored by adding another factor obtained from the same crude culture supernatant. This factor, physicochemically inseparable from serum albumin, was also obtained from the culture medium added with 2‐mercaptoethanol (2‐ME) and incubated at 37°C for 24 hr without the cells. By the experiments using semi‐synchronized cell population, it was demonstrated that 2‐ME or 2‐ME carrying protein acted at the restricted process(es) of cell proliferation which occurred between IL‐2‐acting stage and the ini
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