Incubation of human articular cartilage explants with interleukin-1α (IL-1α) inhibited the rate of [ 3 5 S]sulphate incorporation into glycosaminoglycan (GAG) chains concomitant with an increase in nitric oxide (NO) production. Measurement of the [ 3 5 S]sulphate showed that IL-1α inhibited the synthesis of both keratan sulphate and chondroitin sulphate (CS) chains to a similar extent. This effect was reversed by the NO synthase inhibitor N ω -iminoethyl-l-ornithine (l-NIO). Analysis of alkali borohydride cleaved GAG chains showed that IL-1α had no effect on their size. Similarly when GAG chains were coupled to xyloside the size of the GAG chains attached to the exogenous acceptor decreased but IL-1α had no further effect on hydrodynamic size. IL-1α did, however, inhibit [ 3 5 S]sulphate incorporation into xyloside-linked CS chains. In both experiments l-NIO reversed the inhibitory effect on sulphation. Disaccharide analysis of the [ 3 5 S]GAG chains showed that IL-1α preferentially inhibited sulphation of the 6-sulphated isomer and that l-NIO reversed this effect. Thus, IL-1α-induced NO mediates the inhibition of sulphate incorporation and alters the sulphation pattern of newly synthesised GAG chains.