Glycopeptides were removed by trypsinization from the surface of baby hamster kidney cells (line BHK21-C13), digested by pronase and separated into 2 fractions by exclusion chromatography. The addition of small amounts of either glycopeptide fraction to shaken suspensions of lightly trypsinzied cells inhibited their rapid aggregation, but one fraction was more active than the other and in higher concentrations it was able to inhibit aggregation completely. After this fraction was purified by high-voltage electrophoresis one subfraction also inhibited aggregation. The effect of the glycopeptides increased following their pretreatment with neuraminidase, but preincubation with periodiate or galactose oxidase destroyed all activity. Galactose oxidase also inhibited cell aggregation directly. Similar glycopeptides from virus-transformed BHK21 cells, oligosaccharides and intact and desialysed human urinary glycoproteins had comparatively little or no effect on BHK21 cell aggregation. The results suggest terminal beta-galactosides and possible alpha-galactosides, and to some extent a particular substructure of cell surface heteroglycans are necessary for their inhibitory activity. The parent, plasma membrane of glycoproteins might serve as adhesive binding sites in cell cohesion, but some evidence indicates cell surface sialyl- and galactosyltransferases may not ordinarily act as their complementary binding receptors.

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