AbstractThe species‐specific (gs‐1) determinants of murine and hamster C‐type viruses were detected by direct immunofluorescence (FA) in fixed virus‐infected cells using monospecific guinea‐pig antisera. The interspecies‐specific (gs‐3) determinants were also detected by FA using broad spectrum sera from tumor‐bearing rats.The FA staining of gs‐1 and gs‐3 determinants in fixed infected cells was localized in the cytoplasm as punctate foci at or near the cell membrane and as a diffuse mass at the perinuclear region. The FA staining of live infected cells was limited to antisera with envelope activity.The percentage of FA positive cells in any one cell preparation was inversely related to the efficiency of complete virus synthesis. Blocking experiments support the contention that the gs‐1 and gs‐3 determinants reside on the same molecule with the former occupying a more accessible or dominant position.