Natural antibodies to interferon gamma (IFN-gamma) were found in patients suffering from various viral infections, but also at weak titers in healthy individuals. In the present study we describe a one-step chromatographic procedure for the purification of the anti-IFN-gamma antibodies from human Ig preparations, using a recombinant IFN-gamma-coupled Sepharose CL4B affinity column. The antibodies to IFN-gamma were eluted from the column using 3 different methods without loss of immunological activity. They were found to be Ig, mostly of the IgG1 subclass, and, in the biological assay, to be able to neutralize the de novo expression of Fc receptor sites induced by IFN-gamma on U937 cells.