Lymphocyte transformation test (LTT) was applied to 28 patients who had developed a hypersensitivity syndrome (HSS) during treatment with the antidepressant drug zimeldine. Twenty-seven patients treated with zimeldine without any symptom of an HSS were matched controls. Zimeldine and its metabolites norzimeldine and CPP 200 all induced statistically significant increased [3H]thymidine incorporation in cultured lymphocytes from the HSS patients compared with the controls, norzimeldine being the most potent inducer. The results indicate an immunoreactive process in the development of the HSS induced by zimeldine.