The sensitizing effect of alpha-chymotrypsin was followed up in a group of 116 women to whom parenteral treatment with this drug was associated to antibiotic therapy for various chronic inflammatory gynecopathies. The mean antibody titre to alpha-chymotrypsin, assayed by a passive hemagglutination technique, and the mean intensity of the intradermal tests with this drug were found significantly higher in 15 patients previously treated with alpha-chymotrypsin by comparison with 16 non-treated controls. A significant direct correlation was found between the titre of anti-alpha-chymotrypsin antibodies and the intensity of the intradermal tests. In 12 patients who had not previously been treated with alpha-chymotrypsin the antibody titre and the intensity of the intradermal tests rose significantly after a series of 10 injections with this drug. Eight of 116 patients (approximately 7%) treated with alpha-chymotrypsin developed overt symptoms of sensitization to the drug. The symptoms announcing the sensitization, the relation with the amount of the drug administered and the diagnostic value of intradermal tests are analysed. No relation was found between the presence of atopy and the sensitization to other drugs, on the one hand, and the sensitization to alpha-chymotrypsin, on the other hand.