Morphological transformation and induction of somatic mutation in the hamster embryo cell bioassay have been used to study whether the carcinogenicity of nickel is affected by polycyclic hydrocarbons. The transformation frequency was found to increase with increasing concentration of nickel sulphate, benz[a]pyrene (BP) and methylcholanthrene. In experiments with combinations of nickel sulphate and BP, the transformation frequencies used for all concentrations were higher than for compounds tested separately. The greatest enhancement was found using 5 micrograms/ml NiSO4 . 6H2O and 0.78 microgram/ml BP. The transformation frequency obtained with this combination was 10.7%, compared to 0.5% and 0.6% for the individual substances. No synergistic effect could be detected between nickel sulphate and methylcholanthrene (MC). In experiments measuring somatic mutation by selection for ouabain resistance, the mutation frequency was likewise found to be significantly higher than expected in mixtures of nickel sulphate and BP. The present demonstration of the synergistic effect between nickel sulphate and BP is of interest with the potentiating effect of cigarette smoking on development of lung cancer among nickel refinery workers.