Two IgG1/kappa class monoclonal antibodies specific for human immune interferon (IFN-gamma), designated B1 and B3, were developed. Specific binding of both monoclonal antibodies to natural or Escherichia coli-derived recombinant human IFN-gamma was demonstrated in a solid-phase radioimmunoassay or by immunoprecipitation. Antibody B3 showed potent neutralizing activity against both natural and recombinant IFN-gamma. Antibody B1, which showed neutralizing activity only when very high concentrations were employed, was used for preparing immunosorbents for affinity chromatography of IFN-gamma. When a highly purified preparation of 125I-labeled natural IFN-gamma was loaded onto the affinity column, all of the biological activity was retained on the column. The bulk of 125I-labeled IFN-gamma bound to the affinity column be eluted in biologically active form, suggesting that antibody B1 could be used for the purification of human IFN-gamma. Analysis of IFN-gamma eluted from the column by NaDodSO4/polyacrylamide gel electrophoresis (SDS-PAGE) indicated that both of the known molecular weight subspecies of IFN-gamma (25,000 and 20,000 MW), as well as the presumed dimer of 45,000 MW, were retained by the B1 antibody affinity column.