A study of the accuracy and dependability of the precipitin reaction and factors which may increase these qualities, and the application of an improved technique with a known limit of error to the study of the relationships of certain mammals. This includes an important advance beyond current practice in the use of standard antigen solutions of known protein content, determined by a modified Kjeldahl process. The experimental results deal with the relationships of beef, sheep, goat, pig, horse, dog, human, and various rodent bloods to each other. Both fowls and rabbits were used as precipitin producers and the antisera thus obtained were compared with each other. By such comparison together with reciprocal titrations of many of the antisera, a general agreement was reached as to the relationship of the species concerned. Classification based on adult or developmental morphology seldom lends itself to a quantitative expression. But with the precipitin reaction some general agreement may be reached as to the quantitative relationship of certain forms. Such results would be of particular value in the study of the invertebrata.