Hydrogels of poly(hydroxyethyl methacrylate) (polyHEMA) homopolymer do not normally support the attachment and growth of mammalian cells. By altering the surface it has been possible to dramatically change this cell-substratum interaction so that vascular endothelial cells can attach and completely populate a polyHEMA surface. While this can be achieved by copolymerisation of polyHEMA with methacrylic acid or diethylaminoethyl methacrylate, it is most conveniently achieved by brief treatment of polyHEMA hydrogel with concentrated sulphuric acid. The resultant creation of surface-COOH groups, revealed by electron spectroscopy for chemical analysis, is consistent with the hydrolytic formation of methacrylic acid on the surface layer. Surface--COOH groups created by treatment with chloric or hydrofluoric acids were not effective. Following sulfuric acid treatment, cell adhesion and growth on polyHEMA hydrogel were better than on Teflon and approached those attained on glow-discharge-treated polystyrene. The capacity of acid-treated polyHEMA to adsorb albumin or fibronectin was of the order of 100-fold or 10-fold lower respectively than either polystyrene, Teflon, or segmented polyurethane. Hydrolytic "etching" in this way is proposed as an efficient means of expanding the use of polyHEMA hydrogel as a biomaterial without modifying the overall physicochemical properties of the bulk of the material.