Reticuloendotheliosis virus strain T (REV-T) is a highly oncogenic avian retrovirus which causes a rapid neoplastic disease of the lymphoreticular system. We derived six cell lines (1-3, 1-5, 2-10, 2-14, 2-16, and 2-20) from chicken spleen cells infected with REV-T. These cells can produce both the REV-T and its associated reticuloendotheliosis helper virus, REV-A. Histochemical analyses of these cells indicate that, while they are not stained by benzidine, peroxidase, beta-glucuronidase or acid alpha-naphthyl acetate esterase, they contain a high proportion (95%) of cells positive for acid phosphatase. Light and electron microscopic studies of these cells also revealed morphologies of lymphoblasts or activated lymphocytes with irregular nuclei and dispersed chromatin. Immunochemical analyses indicate that essentially all (90 to 100%) of the cells contain the surface marker Ia, but no cytoplasmic immunoglobulin M and immunoglobulin G could be detected by immunofluorescence staining. Results also show that some of these cell lines contain a low level of terminal transferase (0.02 to 0.17 unit/10(9) cells), and a proportion (3 to 35%) of these cells can be stained by an antiserum directed against chicken bursa cells. These results are consistent with the conclusion that the cells transformed by the highly oncogenic REV-T are lymphoid in nature. In addition, at least some of these cell clones may contain features characteristic of activated B-lymphocytes. Analysis of these cell clones indicates that some cell lines contain an adherent and nonadherent population with some differences in morphologies. In addition, electron microscopic examination revealed that, while the non-adherent cells are actively producing type C viruses, type C viruses are either absent or very rare in the adherent cell populations. These results support the conclusion that some of these cell lines are heterogeneous and contain subpopulations of cells with differences in their ability to produce viruses.