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 the antibodies were bound When CIF was subjected to normal affinity chromatography, specific activity of 1–5 × 106interferon 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 × 10