Studies on lymphocyte stimulation in vitro were carried out with various perspectives. Apart from the well-known mitogens such as PHA and ConA, Candida extract and mercury ions had a relatively strong stimulatory effect, mistletoe extract a moderate one, nickel ions and trichophytin none at all. In malignant lymphomas the degree of stimulation was greatly reduced. In malignant melanomas the degree of stimulation, which at the beginning was often quite low, increased during the course of treatment. There was no correlation between the response to recall antigens and the degree of lymphocyte stimulation. The lymphocyte stimulation test did not prove reliable for the detection of drug allergies.