Abstract Introduction. The selectivity with which antibodies react with the particular proteins used in their production affords a biological method of differentiating proteins which is far more delicate than that of direct chemical analysis. Applications of this method, in one form or another, have been made by various workers in an attempt to determine whether the blood protein, fibrinogen, is an identical protein in all animal species or is in each particular instance a protein peculiar to that species. The conclusions reached by the several investigators are contradictory and the purpose of this paper is to present experiments which offer more extensive proof that fibrinogens are species specific, as are other blood proteins. In 1912 Bauer and Engel (1) reported that beef and swine fibrinogens exhibited individual and species specificity when tested by the method of complement fixation. They also used the precipitin reaction but found that specificity was not so definitely demonstrable by this method.