The reaction of human 5-aminolaevulinate dehydratase with 5,5'-dithiobis-(2-nitrobenzoic acid) (Nbs2) results in the release of 4 molar equivalents of 5-mercapto-2-nitrobenzoic acid (Nbs) per subunit. Two of the thiol groups reacted very rapidly (groups I and II), and their rate constants were determined by stopped-flow spectrophotometry; the other two thiol groups (groups III and IV) were observed by conventional spectroscopy. Titration of the enzyme with a 1 molar equivalent concentration of Nbs2 resulted in the release of 2 molar equivalents of Nbs and the concomitant formation of an intramolecular disulphide bond between groups I and II. Removal of zinc from the holoenzyme increased the reactivity of groups I and II without significantly affecting the rate of reaction of the other groups. The reactions of the thiol groups in both the holoenzyme and apoenzyme were little affected by the presence of Pb2+ ions at concentrations that strongly inhibit the enzyme, suggesting that Zn2+ and Pb2+ ions may have independent binding sites. Protein fluorescence studies with Pb2+ and Zn2+ have shown that the binding of both metal ions results in perturbation of the protein fluorescence.