The principle of specific hapten inhibition is classically used to demonstrate antibody specificity in vitro. The purpose of the reported studies was to investigate whether this principle may also be used to inhibit specifically clinical allergic reactions to penicillin in vivo, in experimental animals and in man. Studies in vitro indicate that a synthetic penicilloyl hapten (benzylpenicil-loylformyllysine, BPO-FLYS) is highly efficient in blocking anti-penicilloyl antibodies, as revealed by inhibition of precipitation, haemagglutination, complement fixation and the Schulz-Dale reaction. BPO-FLYS is also capable of inhibiting in vivo passive cutaneous and systemic anaphylaxis in guinea-pigs and wheal-and-erythema skin reactions to penicilloyl-polylysine in man. Extensive toxicologic studies indicate that BPO-FLYS may be tolerated in large doses in man. Competition experiments in vitro also show that it does not interfere with the antibiotic activity of penicillin.