Summary Six different Rous mammalian tumour lines were studied for the occurrence of virus specific antigens, the presence of the viral genome and the synthesis of infectious and non-infectious particles. Four Rous tumour lines were induced in stu mice presumably free from latent murine leukemia virus (MLV), three of them by a cloned Schmidt-Ruppin strain of Rous sarcoma virus (sr-RSV) belonging to sub-group D, the fourth with an uncloned sr-RSV strain. In addition a Prague(P)-RSV induced mouse tumour line (RVP3) and an sr-RSV induced hamster tumour line (RSH) and sr-RSV induced chicken tumours were likewise investigated. In all mammalian lines, except for RVP3, avian leukosis group specific complement-fixing antigen was found. Tumour specific transplantation antigen was demonstrated in all mammalian lines except RSH, whereas no infectious RSV could be detected in any of the mammalian tumours. Likewise electron microscopy revealed no virus particles in more than 9500 cell sections of RSV-stu tumours, but the viral genome was shown to be present by transfer of the tumour to chickens. In the RVP3 line virus particles were detected resembling typical murine leukemia virus while in the RSH line particles were found morphologically similar to the latent hamster virus described by Bernhard & Tournier (1964). The interpretation and significance of the findings are discussed.