The lymphocyte blastic transformation (TTB), leucocyte and macrophage inhibition tests has been compared in 18 hypersensitive to penicillin patients and in 15 patients with hypersensitivity to PPD. It has been shown that TTB was positive in all penicillin allergy cases, due to the constantly LT sensitization, and therefore this test constitutes an efficient method for the in vitro diagnosis of penicillin hypersensitivity. Macrophage inhibitory factor (MIF) was absent in all penicillin allergic subjects. Leucocyte migration inhibition test was positive only when gamma globulin coupled penicillin was used or in the presence of autologous plasma pretreated penicillin. This suggests that leucocyte migration inhibition resulted from the specific penicillin antibodies.