This work concerns infection of avian erythrocytes by influenza and two other viruses. The first section investigates the production of viral polypeptides and infectious progeny virions from avian erythrocytes infected with fowl plague virus, Newcastle disease virus and Semliki Forest virus. In the second section a closer examination is made of viral polypeptide synthesis in avian erythrocytes following infection with avian and/or human influenza A strains. The third section investigates the distribution of newly synthesized influenza viral polypeptides between the erythrocyte nucleus and cytoplasm. The fourth and fifth sections are concerned with viral replication in erythrocytes at different stages of differentiation.