The occupational and environmental hazards of nickel exposure are of great concern in environmental medicine. Nickel workers have increased risk of cancer of the nose, lung, larynx, and possibly the kidney. In the present investigation we have studied the effects of nickel ions on fetal human kidney cortex explants. The explants were continuously exposed to 5 µg/ml NiSO₄. After 70–100 days in culture foci of phenotypically altered cells appeared. Immortalized cell lines were established and demonstrated to be of human epithelial origin. Tumorigenicity was not induced, but the cells demonstrated decreased requirement for serum, increased plating efficiency and saturation density, and formation of colonies in soft agar. Chromosome changes in the treated cells were observed. Worth mentioning are change in ploidy (3n) and abnormalities of chromosomes 1, 7, 9, 11, 13, 14 and 20; increased numbers of chromosome 17; and loss of normal chromosomes 20 and 22.