To evaluate the specificity of the lymphocyte transformation test (LTT) in the diagnosis of drug allergy we studied over 71 days an atopic woman with a past history of frequent adverse reactions to pyrazolone drugs. Rechallenge with the incriminated substances aminophenazone (aminopyrine) and propyphenazone was carried out on Days 11 and 31 respectively. An immediate type of hypersensitivity reaction was seen after 100 mg aminophenazone, while 100 mg of propyphanozone led to a serum sickness-like syndrome. We found two specifically sensitized lymphocyte populations using either the pure substance or sera containing metabolite in cell cultures. Stimulatory responses with indices ranging between 3 and 6 were seen 3-4 days after exposure, and the tests remained positive in both instances for 3-4 weeks. Specific sensitization was proven by positive skin tests and by a small but distinct lymphocyte proliferative response before challenge. Several lymphocyte function tests were performed over a period of 53 days and revealed a large fall in pokeweed mitogen-induced immunoglobulin synthesis and an increase in suppressor cell activity after rechallenge with aminophenazone. We conclude that the proliferative response observed in the presence of the offending drug is due to the activation of T memory cells and therefore highly suggestive of a true allergic reaction.