RNA tumor virus-like particles in human fetal thymus cells stimulated by human B-cells

Human fetal thymuses were investigated in a search for retroviruses. By electron microscopy, retrovirus-like particles were detected in the thymus cells only when they were cocultured with mitomycin C-treated human B-cells. Reverse transcriptase activity in the culture medium was found at a density of 1.15-1.17 g/cm3 in sucrose density gradients.