Mouse immune interferon was produced from a T-lymphoma cell line, L12-R4, upon stimulation with phorbol myristic acetate at a concentration of 2 x 10(-7) M/ml. The crude interferon, concentrated by precipitation with (NH4)2SO4 at concentration of 80%, was sequentially chromatographed on two sorbents, namely Sephacryl S-200 and Phenyl-Sepharose CL-4B. With molecular sieve chromatography L12-R4 interferon was observed to have an apparent molecular weight of 45.000 d, while from its chromatographic behaviour on Phenyl-Sepharose CL-4B interferon appeared to be highly hydrophobic like other mammalian interferons. The L12-R4 tumor cells therefore provide a unique source of immune interferon for purification and may represent an useful model for studying the molecular mechanisms involved in T cell differentiation leading to immune interferon production.