Macrophage production of arginine-derived NO2- provides a simple method for detection and quantitation of interferon-gamma (IFN-gamma) in murine cell culture fluids. When the macrophage cell line RAW 264 is cultured overnight with IFN-gamma in the presence of 10 ng/ml LPS, NO2- release, as determined by a simple colorimetric assay, is proportional to the concentration of IFN-gamma and is inhibited by monoclonal antibody to IFN-gamma. A high degree of correlation was obtained when antigen stimulated T lymphocyte supernatants were tested for IFN-gamma by ELISA and the NO2- production assay.