A modification of the peroxidase-labelled antibody technique was applied to study the distribution of virus antigens in chicken cells infected with two serotypes (A and B) of avian leukosis virus (ALV). Type-specific chicken antisera reacted only with cells infected with virus of the homologous envelope serotype. When unfixed cells were exposed to type-specific antivirus serum, only antigens located at the cell surface were stained, while cells exposed to type-specific antibodies after fixation revealed both surface and intracytoplasmic virus antigen. Cytoplasmic antigen was usually concentrated in discrete granules which often had a vesicular structure.