Treatment of cultured cells with certain nucleoside analogs (bromoor iododeoxyuridine) can cause the activation of apparently cryptic viral RNA genomes, resulting in the appearance of products varying from special antigens thought to be virally specified (Weiss et al., 1971) up to complete infective type C oncornavirus particles (Lowy et al., 1971; Aaronson etal., 1971) .