Using chromatofocusing, two fractions have been obtained from recombinant-derived interleukin-1 beta (IL-1 beta) and from pituitary-derived bovine growth hormone (BGH). The forms of both proteins responsible for these fractions have been characterized by N-terminal and C-terminal amino acid sequence determination. Recombinant IL-1 beta, as a mixture of correctly processed polypeptide and an N-terminally methionylated form, was resolved rapidly by chromatofocusing. BGH was resolved into the full-length polypeptide commencing Ala-Phe-Pro-Ala-Met-Ser-Leu- and a form truncated at the N-terminus by four amino acid residues, which thus commences Met-Ser-Leu-; the fraction containing the truncated form also contains a species having N-terminal Phe-Pro-Ala-. These results, and the possible generality of the separation, are discussed.