Four different varieties of easily cultivated microorganisms have been cultured from the cerebrospinal fluid and tissues of cases of acute anterior poliomyelitis; namely, a streptococcus, a diplococcus, diphtheroids, and Gram-negative bacilli. It is not contended that they were all inherent in the tissues; a part were doubtless extraneous. The streptococci and diplococd may be considered as the most significant of the bacteria cultivated and are distinguishable from each other by biological tests. The streptococci grew both aerobically and anaerobically; under anaerobic conditions growth was slow, the cocci became small and round, and were more easily decolorized with alcohol in the Gram stain. They were not found in the anaerobic cultures of 106 cerebrospinal fluids; they were found in one of twenty anaerobic blood cultures and frequently in the cerebrum, cerebellum, pons and medulla, cord, tonsils, lungs, liver, kidneys, spleen, pancreas, thymus gland, suprarenal glands, and mesenteric glands of fatal cases. The diplococci are Gram-positive and, transplanted to solid media, yield luxuriant growths and a staphylococcus grouping. They grew aerobically and anaerobically, but more slowly under the latter condition, and the cocci became smaller and more rounded. Diplococci were found in the anaerobic cultures of 48 of 106 cerebrospinal fluids; also in the cerebrum, cerebellum) pons and medulla, cord, tonsils, lungs, liver, kidneys, spleen, pancreas, and mesenteric glands of fatal cases. The filtrates of emulsions of tissues containing streptococci and diplococci passed through fine Kitasato and Pasteur-Chamberland filters were sterile unless large amounts of filtrates were collected. The amount of filtrate collected and cultured is therefore of considerable importance in filtration experiments. The small forms of streptococci and diplococci in old anaerobic cultures are filterable with these filters, while young aerobic cultures containing large forms are not, unless large amounts of culture are filtered. Intracranial, intravenous, and intraperitoneal injection of these easily cultivated streptococci, diplococci, diphtheroids, and Gramnegative bacilli failed to produce paralysis in rabbits or monkeys. With two exceptions all the cultures were transplants from the original anaerobic ascites-broth-kidney cultures of cerebrospinal fluid and various tissues. Arthritis and meningitis were produced by the streptococci, but there were neither clinical iior histological evidences of true poliomyelitis. Occasional bacteriological studies since 1898 have shown that easily cultivated micrococci and bacilli may be present in the cerebrospinal fluid and tissues of the central nervous system of persons suffering with acute anterior poliomyelitis. The majority of bacteriologists have found the cerebrospinal fluid, blood, and nervous organs sterile. Opinions have varied in regard to the significance of the organisms and the micrococci in particular, but the consensus of opinion has been to the effect that they are secondary invaders and unable of themselves to produce poliomyelitis in the lower animals. After allowing for contaminations due to technical errors in securing specimens, the total number of observations indicates that easily cultivated micrococci occur sometimes in the brain and cord of persons suffering from epidemic poliomyelitis. Our studies have shown that they may be found not only in these locations, but also in the spleen, kidneys, suprarenal glands, and other organs. It is not known that they exert an influence in this disease, although they may possibly give rise to the production of antibodies) assuming their entrance not to be wholly agonal,as the cultures of streptococci are frequently of sufficient virulence to produce meningitis in rabbits and monkeys. Our experiments are in accord with those of other investigators who found that these microorganisms do not produce poliomyelitis in the lower animals, and are therefore in sharp contrast with the recent reports which would attribute an etiologic relationship of streptococi and allied organisms to that disease. At present this wide divergence of result cannot be accounted for, but it does not seem that it is possible for it to reside in any condition of the cultures employed by us as they were obtained from undoubted cases of epidemic poliomyelitis and inoculated in early generations. As regards these easily cultivatable microorganisms, we agree at present with those who regard them as secondary and probably terminal invaders rather than the actual etiologic agent of the disease.