RECENT investigations here have shown that rats, monkeys and sheep infected with helminth parasites produce an antibody with many of the properties of human reagin. Naturally occurring persistent antibodies of this type have not been demonstrated previously in animals, although in rats an antibody similar in its biological properties but transitory in appearance can be stimulated when antigen is injected with Haemophilus pertussis killed organisms as adjuvant1,2. Like human reagins3, the antibodies stimulated by helminth infections become rapidly and apparently permanently attached to the skin of animals of the species in which they are stimulated, but they are incapable of sensitizing guineapig skin in passive cutaneous anaphylaxis.