NB4, a maturation inducible cell line with t(15;17) marker isolated from a human acute promyelocytic leukemia (M3). Acute promyelocytic leukemia (APL) is a well-defined entity among acute leukemia, cytogenetically characterized by a t(15;17) (q22;q11-12) translocation. In vitro and in vivo studies suggest that all-trans retinoic acid (RA) treatment restores cell maturation. We have isolated the first permanent cell line with t(15;17), derived from the marrow of a patient with APL in relapse. The establishment of the cell line, its morphologic, karyotypic, and immunohistochemical features are reported. RA induced cell line maturation. Cells strongly expressed myeloid markers, but also some T-cell markers. Additional karyotypic abnormalities, a 12p rearrangement and the possible presence of a homogeneous staining region (HSR) on 19q+ are discussed both in relation to T-cell (CD2, CD4) and monocyte (CD9) markers, and to the acquired cell growth autonomy. The cell line represents a remarkable tool for biomolecular studies. 