1. The cause for the phenomenon of hemagglutination with heated PVM suspensions has been sought.
2. Evidence in wide variety indicates that the component responsible for hemagglutination is the virus particle itself.
3. The virus is capable of combining with a substance present in lung tissue of certain mammalian host species susceptible to infection by PVM. The occurrence of such combination accounts for a number of unusual properties manifested by this pneumotropic virus.