1. From four out of six specimens of saliva from six cases of mumps in the early stages of the disease, a filterable cytotropic virus has been obtained which induces in M. rhesus monkeys, following inoculation of the parotid glands through Stensen's duct, an acute, non-suppurative parotitis analogous to mumps.
2. This virus has not been found in normal saliva, nor does it correspond to any known virus with which we are familiar.
3. The virus is free of demonstrable microorganisms including spirochetes.
4. It is judged that this virus is the causative agent of mumps.