Virus particles, mostly intracisternal, doughnut-shaped ‘A-type’, 90 nm in diameter, resembling those observed in leukemic guinea pig tissues, were observed in placental and fetal, but not adult tissues from normal guinea pigs. Extracellular ‘C-type’ virus particles, 100–110 nm in diameter, with a thick outer envelope surrounding a dense core, similar to those seen in leukemic animals, were found predominantly in BrdU-treated primary cultures prepared from both adult and fetal guinea pig cells. Both types of virus particles were observed after BrdU treatment of guinea pig cell strains or cell lines, whether morphologically transformed or non-transformed, with or without demonstrable tumor-igenicity. Only intracisternal ‘A-type’ particles were found in the two hepatoma cell lines, regardless of BrdU treatment. These two types of virus particles appear to be commonly endogenous in guinea pigs but their interrelationship to each other and to oncornavirus particles observed in other mammalian species is unknown.