Publisher Summary This chapter presents a study analyzing lymphocyte transformation test in allergic contact dermatitis by mercury. Eleven patients (10 female, one male, age range 10–48 years) with contact dermatitis to mercury were studied. All presented with a positive patch test, and erythema, oedema, and vesiculation at 96 hours with mercury metal 0.5% in petroleum. Thirty normal, nonatopic controls with a negative mercury patch test were selected. Lymphocyte transformation test (LTT) was performed on each patient using mercury acetate (Merck) in three different dilutions: (1) 12.5 ng/105 cells, (2) 25 ng/105 cells, and (3) 50 ng/105 cells. Phytohaemagglutinin (PHA) at 10 μg/ml was used as lymphocyte transformation control. Levels of leucocyte histamine liberation (LHL) test were measured at five different dilutions: (1) 500 ng/ml, (2) 200 ng/ml, (3) 100 ng/ml, (4) 50 ng/ml, and (5) 25 ng/ml. All the 30 controls showed nonspecific negative LTT for all concentrations of mercury acetate employed. A response index below 2 was considered negative. Ten of the allergic patients showed responses above 2.