To study the influence of renal polyanions on crystallization of urinary calcium oxalate, we recovered polyanionic macromolecules from media conditioned by primary cultures of renal proximal tubular epithelial cells of rats in serum-free, hormonally defined medium. Cells cultured on microporous supports showed a higher degree of morphological and functional proximal differentiation into a polarized monolayer than those on plastic impervious substrata. Papainization of the polyanions yielded the glycosaminoglycans chondroitin/dermatan sulphate and heparan sulphate. These accounted respectively for 60% and 80% of the crystal nucleation-promoting activities of polyanions recovered from the apical and basal media conditioned by polarized cultures on microporous supports. Similar relative activities were observed among the urinary glycosaminoglycans and polyanions similarly tested. Primary cultures of polarized proximal tubular epithelial cells are useful then as an in vitro model to study the crystallizing activities of polyanionic macromolecules produced by renal cells.