A human lymphoblast cell line, NALL-1, was established from the peripheral blood of a patient with acute lymphoblastic leukemia (ALL). NALL-1 cells had neither properties of T and B cells nor Epstein-Barr virus (EBV). Many characteristics of the NALL-1 line were distinct from those of numerous EBV-positive lymphoblastoid cell lines previously reported. NALL-1 cells are considered to have originated from the donor's leukemic cells on the basis of their cytogenetic, morphologic and functional features. The NALL-1 line is the first human leukemic "null" cell line derived from ALL. The significance of this cell line is disscussed.