Ampicillin belongs to the drugs causing most frequently IgE-dependent allergic reactions, but the current specificity of these reactions is poorly known. Some experimental data suggest that the side chain of ampicillin may induce synthesis of drug-specific IgE antibodies. In our study we have decided to explain the specificity of ampicillin-induced allergic reactions type I appearing in patients after drug administration. Thirty-eight subjects developing an immediate response after administration of ampicillin entered the study. In all the patients skin tests with penicillin, ampicillin and amoxicillin determinants were performed. We have observed positive skin tests results with penicillin determinants in 23 subjects, in 32 subjects with ampicillin and in 17 cases with amoxicillin. Only in three persons the results were positive exclusively to ampicillin. The results of our study indicate that in the Polish population ampicillin induces an immediate IgE response with variable degree of cross-reactivity to other tested drugs, and selective response to ampicillin is uncommon.