Human thymoma is a thymic epithelial cell tumour which often contains a large number of immature T cells and is frequently associated with autoimmune diseases. Since thymic epithelial cells play key roles in the development and selection of T cells in the normal thymus, we hypothesized that the neoplastic thymic epithelial cells of thymoma may support T cell differentiation in the tumour. We characterized CD4-CD8- cells in thymoma and applied an in vitro reconstitution culture system using the CD4-CD8- cells and the neoplastic epithelial cells isolated from thymoma. CD34, a stem cell marker, was expressed on 29.9 +/- 12.2% of CD4-CD8- cells in thymoma. TCRgammadelta was expressed on 27.4 +/- 15.1% of CD4-CD8- cells and CD19, a B cell marker, was expressed on 14.1 +/- 23.1% of CD4-CD8- cells. CD4-CD8- cells expressed both IL-7R alpha-chain and common gamma-chain. Purified CD4-CD8- cells from thymomas were cultured with the neoplastic epithelial cells, and their differentiation into CD4+CD8+ cells via CD4 single-positive intermediates was observed within 9 days' co-culture in the presence of recombinant IL-7. Furthermore, we examined the reconstitution culture using CD34+CD4-CD8- cells purified from normal infant thymus. The CD34+CD4-CD8- cells in normal thymus also differentiated to CD4+CD8+ cells in the allogeneic co-culture with the neoplastic epithelial cells of thymoma. These results indicate that the tumour cells of thymoma retain the function of thymic epithelial cells and can induce differentiation of T cells in thymoma.