Trypsin digestion of Hansenula wingei 21-cells releases a protein (21-factor-T) that inhibits the agglutination of 21-cells by purified 5-agglutinin obtained from 5-cells by subtilisin digestion [Crandall, M. A. & Brock, T. D. (1968) Bacteriol. Rev. 32, 139-163]. We have purified this inhibitor 415-fold by ion-exchange chromatography, affinity adsorption to 5-cells, and gel permeation chromatography. The material shows a diffuse band, on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, with an apparent M(r) of 27,000. It has a pI of 3.8, is rich in acidic amino acids, contains 5% mannose and a trace of glucosamine, and is stable to reducing agents but is inactivated by heat. Zymolyase (beta1-->3-glucanase) digestion of 21-cells releases a similar inhibitor that, after purification, has a larger size than 21-factor-T. This 21-factor-Z appears to contain an additional portion that may serve to anchor 21-factor in the cell wall. Haploid cells of the yeasts Pichia amethionina and Saccharomyces kluyveri also show a constitutive sexual agglutination, and little or no crossreactivity is observed in heterologous mixtures. The agglutination factors in all three genera, however, have parallel properties; one cell type of each pair is heat stable and is inactivated by reducing agents (H. wingei 5-cells, P. amethionina alpha-cells, and S. kluyveri 16-cells), and the other is heat labile and is unaffected by reducing agents H. wingei 21-cells, P. amethionina a-cells, and S. kluyveri 17-cells). Because S. kluyveri 16-cells respond to Saccharomyces cerevisiae alpha-factor with the typical morphogenetic change of a mating half-reaction, the heat-stable agglutinin appears related to the S. cerevisiae a mating type and the heat-labile factor to the S. cerevisiae alpha mating type.