THE immunologic mechanisms of penicillin allergy are of interest to physicians, not only because of their practical value but also because they can serve as a model for the study of the immunologic mechanisms mediating human allergic diseases in general. Untoward reactions to drugs are not all allergic in mechanism, and the same clinical manifestation can have different fundamental causes. As examples, syncope after injection of penicillin may result from a vasovagal fear reaction or from true anaphylaxis, and hemolytic anemia may be due to a biochemical abnormality of the red blood cell (for example, that induced by primaquine) or . . .