Passive anaphylactic shock can be easily produced in mice that have received 250–2,000 million H. pertussis cells four days prior to the administration of 187–200 μ of rabbit antibody nitrogen. Only a few mice died when 108 μ of Ab N were given. In mice receiving 2,000 million H. pertussis cells and sensitized with 187 μ of Ab N an optimal amount of antigen (around 0.04 mg) was found for challenge that if considerably exceeded (5–10 mg/mouse) was actually less effective. Two hundred fifty million H. pertussis cells given four days before administration of 218 μ of Ab N rendered mice highly susceptible to anaphylactic shock, while ten million cells failed to increase their susceptibility. Three to four days after administration of H. pertussis seems to be the optimal time for production of passive anaphylaxis. By the 9th to the 12th day sensitivity has returned to its normal level. After administration of rabbit antibody, sensitivity lasts for three days reaching an optimal point immediately after intravenous injection of the antiserum, and 30 min to 6 hr after intraperitoneal injection. No relationship was obtained between development of anaphylactic sensitivity (active or passive) and development of histamine sensitivity. Adrenalectomy, like H. pertussis treatment, also increases the sensitivity of mice to passive anaphylaxis. Some postulations are made as to the possible mode of action of H. pertussis.