This study seemed of interest to us because of the increasing use, frequently intravenous, of horse serum in large doses, chiefly in the form of curative serums for the infections. Agglutination of the red blood corpuscles of animals by the serum of animals of other species has been extensively studied, especially by Bordet. Some observers also allude to the agglutination of human red corpuscles by horse serum. We have tested nineteen specimens of serum and we have found that it frequently agglutinates human red corpuscles in the test tube.
Nine of these were normal horse serum, only one of them containing a preservative (phenol [carbolic acid] or a cresol). Ten were antistreptococcus serum, antidysentery serum, anti-pneumococcus serum or the like, purchased in the open market or issued by official laboratories, all presumably produced from the horse and usually containing a preservative. The specimens came from six different localities. Some of