The release of the migration inhibition factors, leucocyte inhibitory factor (LIF) and macrophage migration inhibition factor (MIF) from stimulated peripheral blood lymphocytes has been compared in patients with immediate (IgE-mediated) penicillin allergy and in patients with delayed hypersensitivity to tuberculin PPD. It has been shown that in these two groups of subjects, a comparable specific proliferative response can occur following stimulation with the appropriate drug (benzylpenicillin) or antigen (PPD). By cell fractionation studies, the proliferation was found to occur in the isolated T cell population in both subject groups. However, the lymphocyte response to benzylpenicillin was rarely associated with the release of LIF or MIF, in contrast to the situation in tuberculin sensitivity where a concomitant release of LIF and MIF was found. In about one third of penicillin allergic subjects, culture supernatants from specifically stimulated lymphocyte cultures induced migration inhibition in the indirect leucocyte migration test, but the inhibitory activity apparently resulted from the presence of penicillin-specific antibody and not from LIF.