Two E-rosette (spontaneous rosette with sheep red blood cells)-forming lymphoid cell lines were established. One (HPB-ALL) was derived from a young male Japanese patient with acute lymphoblastic leukemia (ALL), and the other (HPB-MLT) was from a 62-year-old female Japanese patient with a leukemic T-lymphoid malignancy. Formation of E rosettes, absence of any immunoglobulin determinants, absence of EBNA (Epstein-Barr virus associated nuclear antigen) and very limited stimulating ability in mixed lymphocyte culture, were characteristics mostly identical with those of so far established T-cell-derived lymphoid cell lines, MOLT, CCRF-CEM, CCRF-HS-B2, RPMI-8402 and JM. Only HPB-MLT, however, has been derived from an aged patient with T-lymphoid malignancy.