In human fibroblasts, cholesterol oxide induced a similar degree of chromosome damage (8.6% of metaphases) and DNA repair synthesis (8-10% of cells with lightly-labelled nuclei) as low doses of ultraviolet light (UV), but did not produce single-strand DNA breaks or DNA damage detectable by inhibition of thymidine incorporation. Chromosome aberrations were detected up to 8 weeks after treatment with cholesterol oxide and UV. Combined treatments had almost additive effects on the frequency of chromosome aberrations but not on repair synthesis. Multiple daily doses of UV did not cause more aberrations than a single dose. Attempts to transform two fibroblast strains from normal donors and three derived from melanoma patients using single and combined treatments of UV, cholesterol oxide and hyperthermia (40 degrees) were unsuccessful.