Articular chondrocytes are known to be phenotypically unstable in culture. One condition that has been reported to suppress dedifferentiation is cultivation at high density on tissue-culture plastic. The aim of the experiments described here was to study the effect of seeding density on chondrocyte proliferation and 35SO4 incorporation, and on the types of collagen and proteoglycan synthesized. I found that cells seeded at low or high density reached the same final density at confluence, and that 35SO4 incorporation, while initially higher (per cell) in high-density cultures, fell under both conditions, reaching the same low level after 3 weeks. The proportion of cells expressing keratan sulphate fell in low- but not high-density cultures and the decline was not prevented by inhibition of cell division. In all the cultures cells expressing keratan sulphate tended to have a rounded morphology. After 21 days in culture, chondrocytes grown at high density expressed predominantly large proteoglycans that aggregated with hyaluronic acid, whereas in low-density cultures a smaller, non-aggregating form was also present. By 21 days in culture cells at both high and low density were expressing type I collagen, although the high-density cells also had an extensive extracellular matrix of type II collagen. These observations support the conclusion that high seeding density stabilizes the chondrocyte phenotype to a greater extent than low seeding density. They also suggest that enhanced dedifferentiation at low density may be due to cell spreading, rather than to selective proliferation of a phenotypically unstable subpopulation of cells.