Reaginic antisera from three species were fractionated by gel filtration. The reaginic activity was localized and compared with other major immunoglobulins in each serum. The reaginic antibody appeared distinct from the major immunoglobulins in sera of man, monkey and dog. Heterologous antisera prepared against the reaginic antibody of all species absorbed reaginic activity. These antisera were prepared in a manner such that they had no detectable activity against the major immunoglobulin classes. These results were interpreted as being consistent with demonstrating similarities between reaginic antibodies of the three species. It is provisionally suggested that reaginic antibodies in sera of dogs and monkeys may reside in a γE analogous to human γE.