A particles with the diameter of 70 to 80 nm were isolated from the cytoplasm of HEp-2, HeLa, and AO cells producing oncornavirus of Mason-Pfizer-like type. Most of the A particles banded at 1.23 to 1.24 g/ml, whereas 3 to 10% banded at 1.29 g/ml in equilibrium sucrose gradients. They banded at 1.30 g/ml in CsCl gradients suggesting that they contained 8% RNA. Individual A particles sedimented at 200 to 250S in velocity sucrose gradients, but their significant part was found aggregated and sedimented at more than 300S. They were resistant to RNase digestion. A particles possessed polymerase activity which was preferentially activated by Mn(2+) rather than by Mg(2+), the RNA template being 60S RNA. Cross-hybridization with two DNA products and immunoassay showed that A particles and Mason-Pfizer-like oncornavirus produced by the same cells contained neither homological RNA sequences nor common antigens, suggesting that A particles are not intracellular precursors of Mason-Pfizer-like oncornavirus but represent an independent oncornavirus. Hybridization of A particle RNA with excess of cellular DNA revealed about 20 proviral copies per HEp-2 cell genome and no proviral copies in human embryo and placenta cell genomes.