beta-Propiolactone-treated (BPL-T) homologous serum albumin caused anaphylaxis in guinea pigs with a frequency and severity equal to that of guinea pigs inoculated with human albumin. Untreated guinea pig serum albumin did not cause any reactions in these animals. Some recipients of current rabies vaccine produced in human diploid cells available in the USA develop systemic allergic reactions, usually following booster immunization. The BPL-T human albumin component of the vaccines was believed to be the cause of the complications. Our studies support this conclusion.