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The identification of Corynebacterium equi, an etiologic agent of pneumonia in foals, offers some difficulty because of the comparative biochemical inactivity of the micro-organism. When encapsulated diphtheroids that produce large moist colonies and a tan pigment are isolated from pulmonary abscesses in horses, there can be little doubt that the bacteria involved are C. equi since the pathological changes produced by the organisms are quite characteristic. When, however, similar bacilli are isolated from aborted equine fetuses, from the genital tracts of barren mares or from various pathological conditions in other species of domestic animals; one hesitates to classify the organisms as C. equi merely on account of morphological and cultural similarity. Inasmuch as this species produces neither acid nor gas from carbohydrates nor gives rise to any other biochemical changes which serve to differentiate it from other chromogenic diphtheroids, its recognition presents difficulties. In an attempt to establish a satisfactory method of identification a serological study of the bacillus was undertaken. Comparatively little work has been done on the serological characteristics of the Corynebacteria. Robinson and Peeney1 summarized the results obtained in the study of C. diphtheriae. These workers found it possible to divide the diphtheria bacillus into a number of agglutinative types which were epidemiologically significant. Brown and Orcutt2 found that the cultures of C. pyogenes which they studied all gave cross reactions in precipitation tests. Antigenic differences were indicated by the intensity of the reactions of certain strains but absorption tests were not performed. Magnusson3 injected two horses with a culture of C. equi. Only one of the horses developed a satisfactory agglutinin titer. All the cultures which he had isolated, as well as a culture isolated by Bull4 in Australia, were agglutinated equally well by the serum of this horse.