The test based on measuring the RNA synthesis rate, already described on Hela S3 in culture (Fauris et al., Les colloques de l'INSERM 106 (1981) 455-463), has been adapted to a human hepatoma cell line Hep 3B which retains a certain capacity towards metabolising. These modifications make it easier to point to the presence of possible water micropollutions and to envisage the study of the medium-term toxicity of such pollutants to be found in traces in water, with adequate sensitivity if we take into account their low concentration level. This trial has been carried out to compare the cytotoxicity of atrazine, over short- and medium-term periods: prolonged exposure to atrazine (1 or 10 micrograms/l) leads to an increase in the RNA synthesis rate (not to be detected after 24 h of exposure). Pre-exposure to 1 or 10 micrograms/l pesticide (over 6 days) would cause results to increase considerably during any future intoxication (250 micrograms/l).