A3/Kawakami was derived from ascitic lymphoma cells of a 68-year-old female patient with malignant lymphoma (non-Hodgkin's lymphoma, diffuse, large cell type) of the stomach and A4/Fukuda was derived from ascitic lymphoma cells of a 52-year-old female patient with double cancer of the colon (well-differentiated papillary adenocarcinoma and non-Hodgkin's lymphoma, diffuse, large cell type). The fresh ascitic lymphoma cells in the case of A3/Kawakami were surface immunoglobulin-positive, but the cultured A3/Kawakami cells no longer expressed any distinct markers. In the case of A4/Fukuda, the fresh ascitic lymphoma cells and cultured cells did not express any specific surface markers. Only 20% of A4/Fukuda cells were reactive with OKI1. However, a small amount of IgM could be detected in the cell extract and concentrated culture supernate of A4/Fukuda. In addition, A4/Fukuda cells heterotransplanted into anti-thymocyte sera-treated newborn hamster or athymic nude mice with a BALB/c genetic background were found to have weak surface immunoglobulin and distinct cytoplasmic immunoglobulin (γ, μ). These data suggest that A4/Fukuda cells share the characteristics of the late differentiation stage of B-cell lineage (intermediate between mature B-cells and plasma cells). It was found that monoclonal antibodies, OKT4 and anti-Leu 3a, which are known to react specifically with inducer/helper T-cells, reacted to both A3/Kawakami and A4/Fukuda cells. The karyotypes of both A3/Kawakami and A4/Fukuda cells were very complicated but included some marker chromosomes such as 14q+.