The lower nasal mucosa and the pharynx of thirty eastern and twenty-three western horses have been examined for streptococci. Eight of the eastern horses carried non-hemolytic streptococci on the nasal mucosa. From the pharynx of six, non-hepiolytic streptococci were cultivated. The throats of eighteen contained strains of the hemolytic type. The nasal mucosa of the eastern horses failed to show hemolytic streptococci. Eight western horses carried non-hemolytic streptococci in the nasal passage; eight also harbored the hemolytic type. Twenty-two strains were isolated from the pharynx. Eleven were hemolytic. Among all the non-hemolytic nasal strains those capable of fermenting mannite predominate. Those of the non-hemolytic types from the pharynx of both classes of horses may or may not ferment lactose but all do ferment either raffinose or inulin. In no instance have any of the non-hemolytic types proved pathogenic for mice. The hemolytic strains from the nasal mucosa of the western horses were all of the Streptococcus py genes type. They were pathogenic for mice and rabbits. One strain from the pharynx of an eastern horse and eight from the throats of the western horses were of the same species. All the others corresponded closely in their fermentation reactions with non-hemolytic streptococci from the same region. The streptococci from pathological sources were all hemolytic. They have fallen into two groups; the larger group (Streptococcus pyogenes) produced acid in dextrose, lactose, saccharose, maltose, milk, and salicin but failed to change the reaction of broth containing raffinose, inulin, or mannite. The streptococci of the smaller group (Streptococcus equi) differ only in their inability to ferment lactose or acidulate milk. Both types are pathogenic for mice. Rabbits are usually more resistant. Streptococcus pyogenes has been isolated from eighteen of twenty-two cases of influenza, three of six cases of strangles, and from eight of nine abscesses. Streptococcus equi was observed in four horses suffering from influenza and five others affected with strangles. This species was also found in an abscess and associated with both rhinitis and pharyngitis.