Antibodies to crude leukocyte interferon (CIF) were bound to Sepharose 4B, and CIF was chromatographed. A relationship between recovered interferon and percentage binding of the antibodies to Sepharose 4B was observed. Highest recovery was obtained when only 50%-85% of th- antibodies were bound. When CIF was subjected to normal affinity chromatography, specific activity of 1-5 X 10(6) interferon units (IFU) (69/19B-units)/mg protein was obtained. When this purified preparation was further chromatographed on a column mainly directed against normal cellular proteins and then concentrated by normal affinity chromatography, a tenfold increase in 'direct' specific activity was obtained. It is suggested that the 'indirect' specific activity is about 2 X 10(8) IFU/mg protein.