Cell aggregation (flocculation) of the yeast Schizosaccharomyces pombe strain RIVE 4-2-1 developed in glucose-containing medium, but only in the presence of ethanol. Cell surface proteins which participated in cell to cell interactions were characterised by the susceptibility of flocculation to different proteolytic enzymes, heat treatment, denaturing and thiol compounds and by the inhibition of flocculation by sugars and derivatives. It was shown that a galactose-specific lectin was involved in this new type of flocculation.