Many procedures have been described for the partial purification of human and animal interferons (for review and additional citations, see Berg, 1982; Pestka, 1981a, b, 1983). Although partial purification and purification of the interferons as bands on SDSpolyacrylamide gels was reported by a number of groups, it was not until 1978 and thereafter that any interferon had been purified to homogeneity in solution in sufficient amounts for its chemical and physical characterization (Rubinstein et al., 1978, 1979a, 1981 a; Stein et al., 1980; Friesen et aL, 1981). The introduction of reverse phase and normal phase high performance liquid chromatography to the purification of proteins (Rubinstein et al., 1979a; Stein et al., 1980; Friesen et al., 1981) lead to the first successful purification of these proteins so that sufficient amounts were available in solution without detergent for their chemical, biological and immunological studies. Various affinity purification techniques particularly antibody affinity chromatography were utilized to purify human leukocyte and fibroblast interferons (Knight, 1976; Berthold et al., 1978; Cabrer et aL, 1979; Zoon et al., 1979; Allen and Fantes, 1980; Okamura et al., 1980; Zoon, 1981a; Berg and Heron, 1981; Kawade et al., 1981; Berg, 1982; Knight and Fahey, 1982; as well as additional references within these citations). This review will concentrate on the purification of the human leukocyte and fibroblast interferons in their natural form as well as the leukocyte interferon produced by recombinant DNA technology. A short review of the purification of human immune interferon is also included.