The efficacy, safety and mechanisms of penicillin desensitization were studied in 24 adults and two children with serious infections that required therapy with a beta-lactam drug. Indications for desensitization included debilitating as well as life-endangering infections. Increasing oral doses of phenoxymethyl penicillin were administered at 15-minute intervals to a cumulative dose of 1.3 million units. Parenteral therapy with the beta-lactam drug of choice was instituted at that point. Immunologic complications of desensitization or therapy, ranging from pruritus to serum sickness, occurred in 12 patients. The appearance of gradually worsening wheezing led to abandonment of the procedure in one subject with cystic fibrosis and severe pulmonary disease. The remaining 25 patients were successfully desensitized and received full-dose parenteral therapy. Chronic desensitization was maintained in seven individuals with twice daily oral penicillins for 3 weeks to more than 2 years. No allergic complications of chronic desensitization or recurrent full-dose parenteral therapy were detected. Skin test reactions to one or all penicillin determinants became negative in 11 of 15 patients retested after acute desensitization. Two desensitized patients became skin test negative, remained skin test negative after cessation of desensitization, and tolerated subsequent beta-lactam therapy without allergic reactions or resensitization. The results of this study provide new evidence that acute and chronic penicillin desensitization is useful and an acceptably safe approach and suggest that antigen-specific mast cell desensitization contributes to the protection against anaphylaxis.