After erythrocytes had been treated with NDV, antigens of the Forssman and mononucleosis type could be demonstrated on their surface. The Forssman antigen was found to be either transfered by the virus to the surface of the erythrocytes or unmasked by splitting off sialic acid from chicken red cells. The mononucleosis antigen could only be transfered to the erythrocytes by NDV. Both antigens demonstrable in NDV, proved to be parts of the normal, host-specific component of the virus. The mononucleosis antigen of NDV was similar to that in bovine erythrocytes, but different from the one in sheep red blood cells.