Mono(2-ethylhexyl) phthalate (MEHP) induced chromosome aberrations in cells of two culture lines, one derived from Chinese hamster ovary cells (CHO) and the other from rat liver cells (RL4). In CHO cells, the clastogenicity of MEHP was unaffected by the presence of an exogenous metabolic activation system (S-9 mix). 2-Ethylhexanol, o-phthalic acid, and phthalic anhydride were without effect. Cytochemical methods and assays for carnitine acetyltransferase and KCN-insensitive palmitoyl CoA oxidation were employed to determine whether chromosome damage was associated with peroxisome proliferation. No evidence of an increase in peroxisome numbers or of induction of marker enzymes was found in CHO cells treated with MEHP for up to 72 hr. Clofibric acid and BR931 were also ineffective. Observations on changes in CHO cell structure and permeability, and on the haemolytic effects of phthalate monoesters, suggest that the cytotoxicity of MEHP may be due primarily to its action on cell membranes. Since chromosome damage was observed only at cytotoxic concentrations, it is suggested that damage to lysosomal membranes and the release of endonucleases may be responsible for the observed clastogenicity of MEHP in vitro.