The authors studied in vitro the variations of erythrocyte ALA D activity in subjects exposed and not exposed to lead risk before and after three tests: heat treatment at 60 degrees C for five minutes, addition of GSH (1 X 10(-2) mol/l) and of zinc (1.5 X 10(-4) mol/l). A study of the ratios before and after treatments showed that GSH and zinc were better than heating in identifying false positives, and that zinc was preferable, owing to lesser dispersion of results around the mean. The ratios of ALA D activity before and after treatment is more closely correlated with PbB levels than ALA D activity without treatment. The addition of GSH restored the enzyme activity of the exposed subjects to values equal to those obtained with heating; this did not happen in the non-exposed subjects. It is postulated that, in lead exposure, the metal acts only on the thiol groups, which may then be reactivated by an optimal concentration of GSH.