Cells of Saccharomyces cerevisiae exhibiting the a mating type secrete into the culture medium a mating‐type‐specific hormone activity (a factor), which specifically causes a transient arrest of DNA replication and cell division in cells of the opposite mating type, α. Three compounds exhibiting a factor activity have been found in culture filtrates from a cells. The most active compound has been purified more than 105‐fold and appears to be homogeneous on the basis of thin‐layer chromatography and thin‐layer electrophoresis in different systems. We propose that this compound, which exhibits in α cells the biological activities that have been attributed to a factor, represents pure a factor. a factor has been characterized as a very hydrophobic undecapeptide with the following amino acid composition: H2N‐Tyr(Asx1, Gly1, Ala1, Val1, Ile2, Phe1, Lys1, Trp1, Pro1). Although in their respective target cells the biological effects of a factor and of α factor, the corresponding mating hormone of mating‐type‐α cells, are remarkably similar, the primary structures of both hormones appear to be quite different.