Summary: Techniques similar to the Seitz filtiation-elution procedure used to purify rabbit-β-lysin have been successfully employed to separate human, horse, and rat β-lysins from other serum constituents. The standard purification procedure for rabbit β-lysin was extended to obtain a more specific antiserum for use in studies on the serologic relationships of β-lysins of different species. Neutralization studies with this antiserum have shown that qualitative serologic relationships exist between β-lysins from the rat, human, horse and rabbit.