Articular cartilage varies in ultrastructure and composition with distance from the articular surface. We have cultured chondrocytes from different zones of pig articular cartilage and investigated whether there are intrinsic differences in their behaviour that might account for the variation observed in intact tissue. On isolation, cells from the upper third of the cartilage were smaller than those of the lower third, but this difference was not maintained in culture. Upper zone cells attached and spread more slowly than lower zone cells; morphological differences between the two populations could be seen for several weeks. The growth rates of the two populations were similar, but upper zone cells reached a lower confluent density. Levels of protein synthesis were similar for both populations, but upper zone cells deposited less proteoglycan in the cell layer. On isolation, the percentage of upper zone cells that stained positive with MZ15, a monoclonal antibody to keratan sulphate, was smaller than the percentage of lower zone cells, but this difference was lost after several days in culture. Nevertheless, the keratan sulphate content of proteoglycan synthesised by lower zone chondrocytes at high density was greater than of that synthesised by upper zone cells. The proportion of nonaggregating proteoglycan was greater in upper than lower zone cartilage and this difference was also observed in long-term cultures. proteoglycans were further characterised by composite and polyacrylamide gel electrophoresis and by immunoblotting; differences detected in cartilage extracts were not, however, maintained in culture; instead, the small proteoglycans synthesised by both upper and lower zone cells varied with plating density. Finally, alkaline phosphatase, a marker of hypertrophic, calcifying cartilage, was only expressed in lower zone cultures. We conclude that some of the observed heterogeneity of articular cartilage reflects intrinsic differences between the cells of different zones, whereas some may reflect the response of chondrocytes to different environmental conditions.