Reversed-phase high-performance liquid chromatography has been used to purify to homogeneity two different lymphokines. Human IL-2 was purified on a C8 reversed-phase column in pyridine-acetate-propanol followed by chromatography on a C18 reversed-phase column in trifluoroacetic acid-acetonitrile. Protein sequence analysis of in situ-generated cyanogen bromide peptides obtained from this preparation established the homogeneity of this material and confirmed the amino acid sequence predicted from the published DNA sequence. Murine CSF-2 alpha was purified on a C18 reversed-phase column in trifluoroacetic acid-acetonitrile followed by chromatography on the same column in pyridine-acetate-propanol. The final preparation yielded a single band on a sodium dodecyl sulfate-polyacrylamide gel with a molecular weight of 24,500.