DNA damage and DNA repair have been observed in cultured human skin fibroblasts exposed to potassium chromate but not to a chromic glycine complex. DNA repair synthesis (unscheduled incorporation of [3H]thymidine (TdR)) was measured in cells during or following exposure to chromate and was significant for chromate concentrations above 10(-6) M. Maximal DNA repair was observed at about 10(-4) M chromate. DNA repair capacity was found to be saturated at this concentration. Chromate was stable for at least 8 h in culture medium and produced approximately a linear increase in repair with duration of exposure. DNA damage as determined by alkaline sucrose gradient sedimentation was detected after treatment for 1.5 h with 5 . 10(-4) M chromate. Exposure to 10(-7) M chromate solution for 7 days inhibited colony formation while acute (1 h) treatment was toxic at 5 . 10(-6) M. The chromic glycine complex was toxic above 10(-3) M for a 1-week exposure but was not observably toxic after a 1-h treatment. These results indicate that chromate and not chromic compounds may be the carcinogenic form for man. The nature of the ultimate carcinogen is discussed. These findings illustrate the utility of the DNA repair technique to study the effects on human cells of inorganic carcinogens and mutagens.