The IgE response, the involvement of the different penicillins available for therapeutic use, and the specificity of the IgE antibodies found in a group of penicillin-allergic subjects from Italy were studied. Thirty subjects with a history of allergic reactions to penicillins were studied. In vivo and in vitro specific IgE antibodies were determined to different penicillin determinants. Fifteen subjects developed anaphylactic responses and the remainder urticaria and angioedema. The drug most frequently involved in the patients’ allergic reactions was ampicillin (AMP). The benzylpenicilloyl (BPO) skin test was positive in 16 (53.3%) patients, whereas 23 (76.6%) patients were positive to minor determinant mixture (MDM), benzylpenicillin (PG), AMP, or amoxicillin (AX). When classified according to initial reaction type, most anaphylactic patients (93.3%) were associated with minor determinant reactivity, whereas most urticaria patients (80%) reacted to BPO. RAST results for the anaphylactic and urticaria subgroups were similar. RAST inhibition showed that most sera contained highly cross-reactive IgE antibodies. There was evidence of a specific response to AX and PG (one patient each). These data show that in a population of penicillin-allergic patients from Italy, AMP was the main drug inducing the allergic reaction. In skin tests and RAST, patients exhibited heterogeneous IgE responses with little indication of specific reactivity to AMP.