Electron Microscopy. Tissues and cell pellets were fixed in 2.5% glutaraldehyde, rinsed with several changes of 0.01 M phosphate-buffered saline, treated with Dalton's chromeosmium, and rinsed again with several changes of distilled, deionized water (6). The specimens were stained overnight with 1% uranyl-acetate in 50% ethanol, dehydrated with increasing concentrations of ethanol, followed by propylene oxide, and embedded in Luft Epon. Thin sections were stained with lead citrate. Negative stains were prepared by mixing isopycnically banded virus in 0.05 M sodium citrate (pH 6.0) with 2% potassium phosphotungstate (pH 4.0). The mixture was deposited on a carbon-coated grid, drained, and air-dried just prior to use. Viral Polymerase Assays. Virus from culture supernatants was concentrated and assayed for viral reverse transcriptase as described (2, 3). Reactions were carried out in a total volume of 0.1 ml using 0.5 Ml of virus (10 Mig of protein) and contained 50 mM Tris-HCI (pH 7.8), 4 mM dithiothreitol, 60 mM KCI, 0.03% Triton X-100, and MnCl2 or MgCl2 as indicated. Po-