1. A pneumotropic virus which forms elementary bodies has been isolated from apparently normal albino Swiss mice.
2. The antigenic relationship of this virus to those of meningopneumonitis, lymphogranuloma venereum, hamster pneumonia (7), and human pneumonitis (8) was established either by cross-immunity or complement fixation or both.
3. In spite of a relationship to other viruses, the virus could be differentiated from all the others studied by certain of its properties.