Penicilloyl-polylysine was used in an intradermal skin test for the detection of penicillin hypersensitivity in 16,239 patients. In 1,003 patients with history of penicillin sensitivity, 396 had a positive skin test compared to a positive test in 775 of 12,559 patients with a negative history. Penicillin was administered to 13,530 patients with a negative skin test, of whom 63 had a reaction. Of 212 patients with a moderately positive and 206 with a strongly positive skin test who were given penicillin, 9 and 21 respectively experienced reactions. No anaphylaxis or life-threatening reactions occurred in patients with negative skin tests, although on the basis of previous experience this number of cases should have produced four reactions severe enough to require hospitalization.