a Toxicity = LD5so of histamine given intraperitoneally, expressed as milligrams of histamine base per kilogram of body weight. b J. Munoz, unpublished data. toxin. This reaction was most probably an anaphylactic shock, although it was not recognized as such by Ospeck and Roberts. Parfentjev et al. (186-188) observed a similar phenomenon when they found that mice pretreated with pertussis vaccine died after receiving a denatured nucleoprotein isolated from B. pertussis. This sensitivity was considered to have an immunological basis, but it was not clearly recognized as being associated with anaphylactic shock until Malkiel and Hargis described their observations on the increased susceptibility of B. pertussis-treated mice and rats to actively induced anaphylaxis (121-124). These observations have been confirmed and extended by various workers employing a variety of antigens (100, 154, 205, 223). Furthermore, B. pertussis-treated mice were also seen to become more susceptible to passively induced anaphylaxis (