Studies of proteoglycans for their capacity to perform the function of steric exclusion of cells are reviewed. The ability of the proteoglycans hyaluronic acid and protein-chrondroitinkeratosulfate and their aggregates to effect cell aggregation and steric exclusion is determined by the structure of glycosaminoglycan components and the amount of covalently bound protein in their macromolecules. These actions of proteoglycans, in which physicochemical processes predominate, can be stimulated or suppressed by protein substances and heparin fractions present in the medium. Results of model tests are fully consistent with those of animal experiments.