We examined the dose-response curves for the induction of morphological transformation of Syrian hamster embryo cells by nickel sulphate or nickel chloride. A low, but significant, induction of transformation was observed at doses of 0.5-5 μg NiCl(2) or NiSO(4)/ml. Increasing the dose to 10 or 20 μg/ml significantly increased the transformation frequency. The increase was not linearly proportional to the increase in dose (e.g. a twofold increase in concentration increased the transformation frequency by tenfold). Treatment with a phorbol ester tumour promoter (12-O-tetradecanoylphorbol-13-acetate, added 2 days after treatment with the nickel compounds) enhanced the transformation induced by nickel at low doses. The possible significance of these findings in relation to epidemiological studies of humans exposed to different levels of nickel are discussed.