Summary: The chicken was shown to be an excellent subject for studies of active anaphylaxis. Anaphylactic shock was best obtained by reinjection on the 6th, 7th, or 8th day following sensitization, and was best avoided by reinjection before the 4th day or after the 15th day. The optimal amounts of sensitizing antigen were 40 to 120 mg protein in animals of about 1500 g; optimal amounts of challenging antigen were 10 to 40 mg protein. A challenging injection of 5 mg was not satisfactory. The frequency of shock increased as the antibody content per ml increased and as the ratio of the antibody content per ml to the amount of challenging antigen increased. No variations in anaphylactic response clearly due to sex or breed were observed.