Natural human interferon gamma(IFN-gamma) was purified from the conditioned medium of peripheral blood leukocytes using selective silica gel adsorption and antibody-affinity chromatography. SDS-PAGE and Western blot analysis demonstrated three major species with molecular masses of 25 kDa, 20 kDa and 17 kDa. Structural analysis of this natural IFN-gamma preparation demonstrated a pyroglutamate residue at the amino terminus and a heterogeneous carboxyl terminus. The longest and most predominant polypeptide was 138 amino acids in length, which is five residues shorter than the sequence predicted from the cDNA. The presence of multiple-carboxyl-terminal forms indicated possible proteolytic processing during induction or protein purification. Limited proteolytic digestion of full-length recombinant IFN-gamma with endoproteinase Lys-C and trypsin revealed that the carboxyl-terminal 15 residues could be released under conditions in which the core portion of the polypeptide chain remained intact. Thus, the heterogeneity of natural IFN-gamma may be explained by partial proteolytic degradation of the molecule and differences in the degree of glycosylation as previously reported [Rinderknecht, E., O'Conner, B. H. & Rodriguez, H. (1984) J. Biol. Chem. 259, 6790-6797].