The T strain of reticuloendotheliosis virus (REV-T) obtained, along with the helper chicken syncytia virus (CSV ), from the CSO 4 cell line was highly oncogenic and rapidly fatal in ducks. Tum ours were m ainly seen in spleen, but neoplastic cells were observed m icroscopically in m any organs. In vitro REV transform ation of duck lym phocytes failed to yield stable cell lines, so cells from organs (blood, bone m arrow, spleen, lym ph node, bursa of Fabricius) of infected birds were used to

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... blish cell lines. Som e of these cell lines have been cloned. The success rates of establishm ent and cloning were increased if cells were cultured in a range of m edia containing different supplem ents; however, m edium containing 5% foetal calf serum (FCS) and 5% duck serum was generally most ef® cacious for initial establishment, while spent medium from the parental line supplemented with a further 20% F CS gave best results for cloning. Cloned cell lines had the m orphology of lym phoblastoid cells, with irregular nuclei and diffuse chrom atin. Analysis of m RNA extracted from these cell lines showed that the uncloned lines were strongly expressing the b chain of the T cell antigen receptor (TCR) and weakly expressing im m unoglob ulin (Ig) polypeptides [l light chain and m , y , y (D Fc) and a heavy chains in various proportions], suggesting the presence of T and B cells. The cloned cell lines that could be classi® ed were TCR b 1 ve T cells. This is the ® rst report of the establishm ent, cloning and partial characterization of duck lym phoblasto id cell lines.