Helianthus tuberosus lectin (HTA) is a mannose/glucose - binding lectin that has a high affinity for α-linked manno - oligosaccharide and branched yeast mannan obtained from Saccharomyces cerevisiae. We report here that HTA could agglutinate S. cerevisiae. The optimum pH of the agglutinating activity was in the range of 6.5 to 8 5. The optimum concentration of HTA for the activity was approximately 1.1mg/ml. When sake yeast strains were treated with trypsin, agglutination of Kyokai strains No.6, 7, 9, 10, 11, 12 and 13 by HTA was remarkably increased, while that of Kyokai strains No.701 and 901 was almost unchanged. The former strains are known to adhere to the surface of air bubbles, but the latter strains do not. In the presence of concanavalin A, snow drop lectin or daffodil lectin, which are known to agglutinate yeast, the trypsin effect was not observed. We discuss the different responses to the trypsin treatment in the two types of yeast strains.