Hunt, D. E. (Charles V. Chapin Hospital, Providence, R.I.) and P. L. Carpenter. Sterols as components in the mating reaction of Hansenula wingei. J. Bacteriol. 86:845-847. 1963.-Agglutinative mating strains 5 and 21 of the yeast Hansenula wingei did not agglutinate after the cells of these two strains were treated with nystatin. The agglutination-inhibiting action of this antibiotic could not be reversed by washing, or heating, or washing and heating the cells. When cells of the two mating types, which had not been treated with nystatin, were heated and washed three times with a chloroform-ethanol (1:3, v/v) solution, the agglutination reaction was reduced significantly. If unwashed cells of mating strain 5 were tested for agglutination with chloroform-ethanol washed cells of mating strain 21, normal agglutination occurred readily. The reciprocal of this agglutination test also resulted in normal agglutination. Evidence is presented that the factor(s) required for the mating reaction in H. wingei can be supplied by either strain, and sterols or related compounds have an active role in the mating reaction.