Libecillide is a specific monovalent penicilloyl hapten inhibitor of allergic reactions to penicillin. A depression of skin hypersensitivity to PPL and/or penicillin and penicillin derivatives sometimes persisting for weeks and months was obvious in numerous allergic patients submitted to combined libecillide-penicillin treatment. A depressing effect on antipenicillin antibody titers detected by passive hemaglutination was also manifest in some patients. The overall tolerance of libecillide in allergic patients has been very good. Nevertheless, the major obstacle to a wider general use of libecillide at the present time appears to be the occurrence of positive skin reactions to that compound in approximately 5 percent of patients allergic to penicillin.