The molecular structure of alpha factor, a mating hormone produced by alpha mating type cells of Saccharomyces cerevisiae has been investigated. From culture filtrates of alpha cells four oligopeptides exhibiting alpha factor activity have been isolated. These peptides, designated as alpha1, alpha2, alpha3, and alpha4 are structurally closely related, being composed of thirteen (alpha1 and alpha3) and twelve (alpha2 and alpha4) amino acids, respectively. The peptides were found to be composed of the following amino acids: 2 glutamic acid or glutamine, 2 proline, 1 glycine, 1 methionine or methionine sulfoxide, 2 leucine, 1 tyrosine, 1 lysine, 1 histidine, 1 or 2 tryptophan. The tridekapeptides differ from the dodekapeptides by an additional NH2-terminal tryptophan residue. Tyrosine was identified as the C-terminal amino acid in all four peptides. alpha3 and alpha4 are oxidation products containing an internal methionine sulfoxide instead of methionine. The mechanisms which could introduce the observed heterogeneity of the peptides are discussed.