Clavulanic acid is a potent inhibitor of B-lactamase that is increasingly prescribed in association with amoxicillin. We report 2 cases of patients who experienced pruritus, wheals, and angioedema after oral intake of amoxicillin/clavulanic acid. Routine skin tests for B-lactam antibiotics and specific immunoglobulin (Ig) E were negative in both patients. Analysis of CD63 expression by the basophil activation test (BAT) using flow cytometry and of sulphidoleukotriene (sLT) release by basophils using the cellular allergen stimulation test (CAST) revealed significant positive responses with amoxicillin/clavulanic acid and with clavulanic acid, and negative responses with amoxicillin and other beta-lactam antibiotics. In addition, cultured CD3+CD4+ cells showed a significant increase in the expression of CD69, CD25, and HLA-DR in the presence of clavulanic acid. Both patients tolerated therapeutic doses of amoxicillin. BAT and CAST are useful ex vivo procedures for the detection of specific IgE-mediated allergy to clavulanic acid, especially for patients with negative skin test results.