The lymphocyte proliferation test (LPT) was compared with the patch test in the diagnosis of nickel contact sensitivity. Of the 21 subjects with nickel contact allergy, the patch test detected 20 (95%). The subject remaining negative in the patch test was positive in the intradermal test. 18/21 subjects with nickel contact sensitivity were positive in the LPT, whereas in the control group 2/23 subjects were false positive. These results were obtained at nickel sulfate concentrations of less than 10 micrograms/ml, higher concentrations led to nonspecific lymphocyte stimulation. 3 nickel-sensitive cashiers with suspected coin contact-induced deterioration of their hand eczema were challenged by having them count nickel-containing coins daily for 15 min. 2 of them developed vesicular eczema on their palms and fingers in 2 to 3 days. The present results show that the LPT is a reliable additional test in the diagnosis of nickel contact sensitivity. Furthermore, provocation is a valuable procedure when assessing the relevance of nickel contact in occupational hand eczema in certain occupations.