Human interferon prepared by challenge of leukocytes with Sendai virus, or of fibroblasts with double-stranded poly(inosinic acid).poly(cytidylic acid), has been studied with respect to purification by affinity chromatography. Both leukocyte and fibroblast interferons are removed from crude tissue culture fluids by means of columns of antibody to leukocyte interferon attached to Sepharose-4B. The antibody was prepared in sheep using, as antigen, material that had been partially purified by gel filtration through Sephadex G-100 columns. Many of the impurities in the crude fibroblast interferon were presumably not recognized by the sheep antibodies induced by leukocyte interferon. Fibroblast interferon was, therefore, much more effectively purified as the result of this "common denominator" approach. The fibroblast product, in contrast to interferon from leukocytes, could only be harvested efficiently from the crude starting material when a carrier protein (bovine-serum albumin, and later, cytochrome c) was added to the eluting buffers to counteract losses, presumably due to adsorption on purification and assay equipment. Both varieties of interferon exhibit molecular weights of approximately 20,000-25,000, although association with higher molecular weight proteins occurs.