A lymphocyte transformation test (LTT) was able to distinguish between nickel-sensitive subjects and non-nickel-sensitive controls. Sixty-one out of 66 (92%) nickel-sensitive subjects had positive stimulation indices in 6- and/or 7-day assays using 5 micrograms/ml of nickel sulphate, whereas none of the 43 controls gave positive results. Stimulation indices were not enhanced by the patch testing of subjects to nickel before performing the LTT. A weak correlation was seen between the results of the LTT and the macroscopic degree of patch-test reactivity. The concentration of nickel sulphate used (5 micrograms/ml) did not have a significant non-specific mitogenic effect.