1. There is present as a causal agent in the nasopharyngeal secretions of measles cases a filter-passing virus.
2. The rabbit and the guinea pig react specifically to the intratracheal and intracirculatory injections of filtered nasopharyngeal secretions secured from cases of human measles.
3. Enanthem, exanthem, and pyrexial disturbances characterize this specific reaction in the rabbit; in the guinea pig the reaction manifests itself by pyrexia, marked leucopenia, and grave nephritis in the fatal cases.