A non-haptoglobin alpha 2-globulin was observed in the plasma of goats (Capra hircus) after inflammatory stresses. 2. This stress reaction protein exhibited an electrophoretic mobility and molecular weight similar to goat haptoglobin. 3. Complete immunological cross-reactivity was demonstrated between the goat protein and human alpha 2-macroglobulin. 4. The functional and evolutionary significance of these observations are discussed.