The relationship between antioxidant-enzyme defence responses and cellular growth suppression in human MCF-7 (breast) and SW480 (colon) cancer cells, exposed to CLA in culture was studied. MCF-7 and SW480 cells (1 x 10(6)/flask) were cultured in appropriate medium for 4, 8 and 12 days with varying levels of CLA (0-30 ppm). A dose-dependent decrease in cell numbers and increase in lipid peroxidation, as determined by thiobarbituric acid reactive substances (TBARS) was observed in both cell lines following incubation with CLA. Exposure of both cell lines to 20 ppm CLA for 2-6 days produced a reduction (83-91%) in 3H-leucine incorporation into protein while 3H-uridine and 3H-thymidine incorporation into RNA and DNA were reduced by 49-91% and 86-98%, respectively, compared with untreated control cells. The activities of superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx) were induced in both cell lines exposed to CLA (20 ppm) over a period of 12 days, although to a greater extent in MCF-7 cells than in SW480 cells. The data indicate that CLA-induced cytotoxicity against MCF-7 and SW480 cancer cell lines is related to the extent of lipid peroxidation of CLA treated cells and affirm that the CLA- induced antioxidant enzymes failed to protect these cells from cytotoxic lipid peroxidation products.