The mutagenic activities of the chlorinated butenoic acids recently identified in chlorinated drinking waters were determined by the Salmonella microsome assay and by the SOS chromotest. The Salmonella typhimurium tester strains TA97, TA98 and TA100 were used without and with S9 mix. In the SOS chromotest Escherichia coli PQ37 was used as an indicator organism with and without metabolic activation. In addition, the extremely potent Ames test mutagen (Z)-2-chloro-3-(dichloromethyl)-4-oxobutenoic acid (MX, in the open form), was studied by the micronucleus test with mice using intraperitoneal treatment. The results of the Salmonella assay and the SOS chromotest showed that MX was by far the most potent mutagen of the compounds tested. Mutations were also induced by the reduced form of MX, (Z)-2-chloro-3-(dichloromethyl)-4-hydroxybut-2-enoic acid (red-MX), and by the geometric isomer of MX, (E)-2-chloro-3-(dichloromethyl)-4-oxobutenoic acid (EMX). However, since the solution of EMX contained approximately 5% MX, most of its activity might be attributable to MX. The oxidised form of EMX, (E)-2-chloro-3-(dichloromethyl)-butenedioic acid (ox-EMX), was marginally active in the SOS chromotest only. All these compounds were directly acting mutagens and in the presence of metabolic activation (S9 mix) they did not generate mutagenicity. The oxidised form of MX, (Z)-2-chloro-3-(dichloromethyl)-butenedioic acid (ox-MX), was not mutagenic at the dose levels tested. MX did not induce micronuclei in the bone marrow of mice.