In 1923 Orskov demonstrated the presence of branching organisms in the human pharynx. This discovery depended mainly on direct microscopic observation of growing cultures on agar; in making smears in the usual way the branchings are easily broken up. The present report, which is a thesis for the doctor's degree, presents the results of an extensive study of this group of organisms, 130 strains of which were isolated from the pharynx and nasopharynx. The culture and other characteristics of the organisms, which appear to be nonpathogenic, are described. An interesting observation is that most of the strains, especially those belonging to the acid group, produce the enzyme which by its action on human red blood corpuscles renders them agglutinable to human serum in general, including the serum of the person whose corpuscles are examined (the so-called T agglutination described by Thomsen).