IgE was detected in sera of cows previously inoculated with ovalbumin in alum or in Freund''s incomplete adjuvant. Titers > 64 were produced by homologous passive cutaneous anaphylaxis (PCA) reactions on calf skin, using sera from these cows. This bovine IgE was heat labile. Bovine sera containing IgE were used to examine cross-species reactivity by heterologous PCA. Rats, rabbits, guinea pigs and dogs failed to develop PCA reactions when sensitized by intradermal injections of the bovine IgE and challenge exposed at intervals from 2-72 h later with i.v. ovalbumin and Evan''s blue dye. Sheep and goats became passively sensitized by the bovine IgE. Considerable variation in latent period and titer was apparent after comparison of results in heterologous species with each other and with the homologous recipient. The ability of this homocytotropic antibody to cross species is important to the phylogenetic relationship of IgE in different species.