Abstract
Chicken cells infected with avian RNA tumor virus often contain small cytoplasmic A-type particles which commonly exist as clusters of 50--100 particles when viewed in thin sections. These particles were found more consistently in Rous sarcoma virus-infected than Rous-associated virus-infected cultures, but were generally present in only a small fraction of the total infected cells. The results of the survey of cells infected with various strains of leukosis-sarcoma viruses led to the hypothesis that the A particles develop in cells undergoing cytopathological degeneration. The hypothesis explains also the evanescent nature of the appearance of these particles in infected cells. The application of immunoelectron microscopic methods using monospecific antisera against viral internal proteins revealed that the A particles contain components immunologically related to the proteins of C-type virus.