A reference value for the nickel concentration in finger nails from people who are not occupationally exposed to nickel was determined on the basis of nail samples from 95 healthy individuals. The mean +/- standard deviation was 1.19 +/- 1.61 mg/kg and the median was 0.49 mg/kg (range 0.042-7.50 mg/kg). The 95% confidence interval of the population mean was 0.51-1.26 mg/kg and the 95% confidence interval of the population median was 0.39-0.74 mg/kg. The random sample consisted of 59 women and 36 men. No significant difference between the nickel levels in finger nails from men and women could be demonstrated. The age of the subjects varied from 16 to 66 years. There was no correlation between age and nickel concentration in finger nails.