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1. Analysis of reticulocytes for δ-aminolaevulinic acid synthase (AmLev synthase, EC 2.3.1.37) and haem synthase (EC 4.99.1.1) activity in 17 workers in pulp production with low-level hydrogen sulphide and methylmercaptan exposure showed decreased activities in eight and six cases respectively. 2. Erythrocyte protoporphyrin concentration was below the control range in seven cases. 3. Low AmLev synthase and haem synthase activities were found in one patient with hydrogen sulphide intoxication 1 week after the event. The activities had returned to the control levels 2 months later, though erythrocyte protoporphyrin remained abnormally low. 4. In vitro, hydrogen sulphide inhibited haem synthase with an apparent Kl of 3.4 mmol/l. Sulphide anion, on the other hand, inhibited AmLev synthase activity 85% at 10 mmol/l concentration. Thiosulphate anion inhibited AmLev synthase activity 18% (Kl 27 mmol/l) and haem synthase activity 43% at 10 mmol/l concentration. Selenite inhibited AmLev synthase (Kl 5.1 mmol/l) and haem synthase (Kl 9.0 mmol/l). 5. The assay of AmLev synthase and haem synthase could be a valuable addition to the assessment of workers' health in industries generating hydrogen sulphide or/and methylmercaptan, although the mechanism of the toxic effect remains speculative.