Abstract The agglutination factor from mating type 5 of Hansenula wingei was fractionated by various techniques. The factor was heterogeneous in chromatography on phospho-cellulose and notably broad in its sedimentation distribution diagram. Highly active fractions could be obtained either from chromatography on phospho-cellulose or from fractions sedimenting at rates greater than 100 Svedbergs. The most active fractions had particle weights greater than 10 8, and only a few of these particles per cell were required to agglutinate the opposite mating type. The agglutinating factor appears to be a protein-mannan complex.