gamma (immune) interferon (IFN-gamma) was induced in cultures of fresh human lymphocytes by combined treatment with a phorbol ester (12-O-tetradecanoylphorbol 13-acetate, TPA) and the T cell mitogen phytohemagglutinin (PHA). Compared to the IFN-gamma yields obtained with PHA induction alone, the inclusion of TPA caused a significant enhancement of IFN-gamma production. Poly(A)-containing mRNA was isolated from mononuclear cells induced with TPA and PHA. Injected into Xenopus laevis oocytes, this mRNA preparation gave rise to IFN activity with characteristic properties of human IFN-gamma. Sucrose density gradient centrifugation analysis showed that IFN-gamma mRNA sedimented at 15 S, suggesting that it contains a total of about 1400 nucleotides.