Human interleukin 2 was separated into multiple molecular forms by selective immunoaffinity chromatography and chromatofocusing. For the most part, this heterogeneity was attributed to variations in glycosylation of the threonine residue in position 3 of the polypeptide chain. The various molecular forms of interleukin 2 had nearly identical specific activities in the in vitro proliferation assay, indicating that the glycosylation had no significant effect on this response. The entire primary sequence of interleukin 2, including the location of the intramolecular disulfide bridge, was determined by a combination of peptide mapping and protein sequencing. This information should aid in the determination of the active site(s) of the molecule.