Extracts from interferon-treated human cells show an enhanced level of a double-stranded RNA-dependent protein kinase activity that is manifested by the phosphorylation of an endogenous Mr 69,000-72,000 protein in its phosphate-saturated state. By using a highly purified protein kinase fraction from interferon-treated human Daudi cells, we can now describe the preparation of murine monoclonal antibodies directed against this phosphoprotein, the Mr of which in its native state is found to be 68,000. These monoclonal antibodies (class IgG1) can identify the electrophoresed protein (p68) in polyacrylamide gels by the electrophoretic transfer blotting technique. Immunoprecipitates formed after incubation of extracts from interferon-treated human cells with the monoclonal antibodies can be conveniently recovered by protein A-Sepharose. Such immune complex preparations have associated protein kinase activity--i.e., addition of [gamma-32P]ATP results in the phosphorylation of p68 and added substrates, calf thymus histone, and eukaryotic initiation factor 2. Immune complex preparations from [35S]methionine-labeled extracts show the specific immunoprecipitation of p68. In addition, several other [35S]methionine-labeled proteins are bound unspecifically in these immune complexes prepared under similar experimental conditions as for the assay of protein kinase activity. These unspecifically bound proteins can be washed out by using a buffer containing detergents or high concentrations of KCl and magnesium acetate. Immune complex preparations washed similarly with these buffers still retain p68 but lose their capacity to phosphorylate p68 or exogenous substrates. These results indicate that p68 by itself has no protein kinase activity. The induction of [35S]methionine-labeled p68 in Daudi cells occurs with as little as 1 unit of human alpha interferon, with maximal synthesis between 6 to 9 hr after the addition of interferon. Actinomycin D blocks this induction.