The potential of the pesticide trichlorfon to induce mitotic aneuploidy has been investigated in genetically engineered human lymphoblastoid cell lines. Trichlorfon failed to induce micronuclei in the AHH-1 and MCL-5 cell lines when treated in media at normal cell culture pH (pH 7.3). Under a treatment pH of 5.5, trichlorfon exposures resulted in the induction of both chromosome loss and chromosome non-disjunction as measured by fluorescence in situ hybridisation (FISH) using a pan-centromeric probe for all human centromeres and centromere probes specific for chromosomes 2, 7 and 18. At treatment concentrations greater than 20 micrograms/ml trichlorfon also induced structural chromosome damage resulting in the production of centromere negative micronuclei.