Cysteine dioxygenase (CDO) is the initial and rate-limiting enzyme involved in the oxidative degradation of cysteine to inorganic sulphate. It is believed to be the major source of sulphate in vivo. Inflammatory conditions such as rheumatoid arthritis have been linked with high plasma cysteine:sulphate ratios in patients. The cytokines tumour necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) have been shown to inhibit the expression of CDO in neuronal (TE671) and hepatic (Chang) human cell lines at nanomolar concentrations. Cytokine release may therefore modulate sulphate production and hence regulate formation of sulphated biocomponents.