We recently described an original epidemiological form of bovine leukosis in cattle. In the young female offspring of one bull, more than 3% of animals developed a thymic lymphoblastic lymphosarcoma. Of these, 31 cases, together with a lymphoid cell line established from one of the tumours, were phenotypically characterized. Characterization was done using a large combination of well clustered monoclonal antibodies, and monoclonal antibodies prepared in our laboratory by immunizing mice either with bovine normal lymphocytes or with tumour thymic cells. The thymic tumours and the cell line did not express any T lymphoid antigens but they did express some B lymphoid markers. The phenotype of the tumour cells was CD45+/-, CD44+/- TdT+, class II-DR+/-, CD19+/-, CD21-, Ig- and HBM 57+ (recognizing the mb-1 chains of the B-cell receptor). The cell line expressed a more mature phenotype: TdT-, CD45-, CD44+, class II-DR+, CD19+, CD21+/- and sIgG+. These results allow us to consider these tumours as B-cell derived. These B lymphosarcomas with a thymic localization are reminiscent of a human mediastinal non-lymphoblastic lymphoma reported as a primary mediastinal clear cell lymphoma. The possibility of a thymic or extrathymic origin for this B lymphosarcoma is discussed.