Evidence has been presented to show that concentrated measles virus in presence of monkey red blood cells will demonstrate the phenomen of hemagglutination. Baboon kidney propagated measles virus yielded higher levels of hemagglutinin than measles virus grown in other primate tissue. Chick tissue propagated measles virus did not demonstrate hemagglutination. Specific inhibition of hemagglutination was demonstrated by rabbit and guinea pig antisera prepared against measles virus in our laboratory and in another laboratory, by naturally occurring simian measles antisera, by human gamma globulin and by human convalescent measles antisera. Inhibition was not observed when measles CF negative simian sera, measles acute human sera and a number of viral antisera were tested.