Leukaemia inhibitory factor (LIF) is a secretory glycoprotein produced by tumour, mesenchymal and haemopoietic cells. LIF has been found to have pleiotropic actions that include the capacity to regulate cell differentiation, promote acute-phase protein synthesis and stimulate calcium release in bone explants. In view of its similarity to other cytokines that affect cartilage metabolism, the effects of LIF on proteoglycan resorption were examined in pig cartilage explants. Endotoxin-free recombinant mouse LIF was found to produce a dose-dependent increase in sulphated glycosaminoglycan (S-GAG) release (ED50 = 123 U/ml, approx. 25-50 pM). Statistically significant stimulation was observed with doses of 100 U/ml or greater. When pig cartilage was stimulated with maximum concentrations of LIF and either interleukin 1 alpha (IL-1 alpha), interleukin 1 beta (IL-1 beta) or tumour necrosis factor alpha (TNF alpha), in each case a significantly greater release of S-GAGs was observed than with the respective cytokines alone (P < 0.05). Comparison of the areas under the curves showed that the action of LIF was additive, and not synergistic with other catabolic cytokines. Dose-response studies showed that transforming growth factor beta (TGF beta) produced a partial inhibition of LIF-stimulated release of S-GAGs (ED50 = 4.5 U/ml). Statistically significant inhibition was observed with doses of 2 U/ml or greater. These results showed that LIF stimulated proteoglycan resorption in vitro and that this effect was modulated by other cytokines. Whether LIF contributes to the progressive destruction of cartilage in septic or chronic inflammatory arthritis remains to be determined.