Lymphocytes from 8 patients with contact dermatitis and a positive nickel patch test, 7 patients with contact dermatitis due to other factors and with a negative nickel patch test, and 9 other subjects, 7 of whom suffered from other dermatological disorders, were tested with the lymphocyte transformation test (LTT), using nickel sulphate in various concentrations. All execpt one of the nickel allergics showed a significant response to nickel, whereas the controls showed borderline (3 patients) or no response (12 patients). These observations confirm previous findings indicating that nickel hypersensitivity can be diagnosed in vitro. This may be of importance in cases of acute contact dermatitis where patch testing is undesirable. Some individuals with a negative nickel patch test responded significantly though weakly to nickel in the LTT, indicating that apart from acting as a specific antigen (hapten?) nickel sulphate may also have weak non-specific mitogenic properties.