The in vivo effect of Inmunoferon (AM-3), a glycophosphopeptide of fungal origin, has been studied on T and B lymphocyte mitogenesis, Interleukin 2 (IL-2) synthesis and natural killer (NK) activity. Inmunoferon (30 mg/kg/day) was administered to several groups of mice 2, 3 or 7 days/week for 2 weeks, and its effect assessed on day 15 of treatment. Every treatment assayed enhanced IL-2 and NK activity in the spleen. The greater effect was produced by daily administration of the immunomodulator. No enhancement was found in mitogen-induced proliferation of T and B lymphocytes. Similar treatments with Inmunoferon enhanced NK activity in the spleens of mice treated with 250 mg/kg cyclophosphamide. In addition, mitogenic responses of T lymphocytes, but not IL-2 production, were also increased in immunosuppressed mice after treatment with the immunomodulator.