The murine Mx-1 protein is one of the best biochemically and functionally characterized interferon (IFN)-induced proteins that is necessary, and sufficient, for providing resistance to murine cells against viral influenza infection. Recently an intracellular human protein homologous to the murine Mx-1 protein has been identified by means of a specific monoclonal antibody. The restricted induction of this intracellular protein in human mononuclear cells (MNC) by various cytokines was investigated. MNC from 26 of 28 healthy people and 35 of 36 cancer patients before IFN-alpha therapy had no detectable Mx-homologous protein. Incubation of human MNC with IFN-alpha and IFN-beta for 24 h at different concentrations led to a dose-dependent induction of the Mx-homologous protein. All IFN-alpha or IFN-beta preparations tested were equally effective in eliciting this intracellular protein. IFN-gamma induced only 1% of the Mx amount elicited by type-1 IFN compared on a weight basis. Neither interleukin (IL) 1 nor IL3, IL4, IL5, IL6, tumor necrosis factor-alpha/beta, granulocyte colony-stimulating factor (CSF) or granulocyte macrophage-CSF at any of the concentrations tested were capable of eliciting any detectable amount of the Mx homolog, while IL2 was a poor Mx-homologous protein inducer. In the presence of high-titered IFN-alpha antisera both IL2 and IFN-gamma were unable to stimulate this protein, proving that IFN-gamma and IL2 indirectly induce the Mx homolog via IFN-alpha. Therefore, the human Mx-homologous protein is a strictly by type I IFN-regulated protein in human peripheral blood lymphocytes.