In contrast to the well characterized serum stimulators of cartilage metabolism, information is scarce on the nature of circulating inhibitors. Human serum was fractionated by molecular sieving chromatography on Sephadex G-200, G-75, G-50 and Biogel P-4 gels. Six fractions with molecular weights of 150, 45, 30, 16, 9 and 1.2 kD inhibited [35S]sulphate incorporation into rabbit cartilage segments. All fractions but the smallest one exhibited their inhibitory effect only in the serum-stimulated cartilage. The 1.2 kD fraction impaired [35S]sulphate and [3H]methyl-thymidine incorporation into the cartilage segments in both stimulated and basal conditions. A seventh inhibitory fraction corresponded to the serum salt peak.