Decreased erythrocyte glutathione s-transferase activity was found in workers exposed to hot rubber fumes in two-roll mill and calender departments (6.6 +/- 0.9 and 6.8 +/- 0.8 mumol/min X mg haemoglobin, respectively vs 8.4 +/- 0.5 in controls). This trend was also noted in the general category of all rubber exposed workers without stratification according to their task (7.6 +/- 0.5, n = 30). This result was contrary to that expected, as the cases with the lowest values had the greatest urinary thioether excretion. The thioethers are the end-products of the enzyme-catalyzed glutathione conjugation reaction aimed at the inactivation of potentially toxic chemicals. Thus, it appears that the assay of erythrocyte glutathione s-transferase activity is not very informative about exposure, although the enzyme assay might disclose other, so far unknown, biological effects of reactive rubber chemicals.