The immunological properties of the surface of chick embryo cells infected with strain MC29 avian leukosis virus were investigated by immune electron microscopy in conjunction with antiviral and anticellular immune sera. When samples were taken at sequential times after infection, it was found that cells stained strongly for viral antigen very early after infection. Staining reached a minimum 6 to 12 hr after infection and then increased to maximum at 15 hr after infection, remaining at this level at later times. Anticellular immune sera showed the presence of a distinct antigen or antigens which appeared concurrently with morphological alteration of the infected chick cells. This antigen was cross-reactive or identical to an antigen appearing on the surface of chick embryo cells transformed by the Schmidt-Ruppin strain of Rous sarcoma virus.