Interferon production in cocultures between lymphoma cells and lymphocytes was previously found to be caused by mycoplasma contamination of the tumor cells. Here we have shown that M. arginini caused interferon production in spleen cells of C57BL/6 mice. The producer cell in spleen cell cultures was insensitive to treatment with anti-theta serum plus complement and it was present in spleen cell cultures of homozygous nude mice. Macrophage depletion did not adversely affect interferon production whereas interferon production was abolished in cultures of cells obtained after passage through nylon wool columns. Collectively, these data suggest, but cannot definitively prove that interferon is produced by B cells. By the use of specific antisera it was found that by all probability the interferon produced represented interferon alpha.