1. By means of differential ultracentrifugation, a purified and concentrated macromolecular fraction has been regularly obtained from infected human, monkey, and chimpanzee stools. This fraction was isolated from sixteen stools in which virus was thought to be present, and inoculated intracerebrally into sixteen monkeys, of which fifteen developed poliomyelitis. 2. Eleven stool specimens in which virus was suspected, when tested separately in eleven monkeys by the intra-abdominal-intranasal method, produced poliomyelitis in two of these animals. When the same specimens were tested separately by the ultracentrifugal-intracerebral method, poliomyelitis developed in ten monkeys out of eleven inoculated. 3. With the intra-abdominal-intranasal method, it has been customary to inoculate the virus present in 1.6 gm. of stool. With the ultracentrifugal-intracerebral method, the virus present in as much as 30 gm. of stool has been inoculated. 4. From one titration experiment it would appear that the ultracentrifugal-intracerebral method is at least 100 times more sensitive than the intra-abdominal-intranasal method.