A number of human hematopoietic cell lines representative of the neoplastic cell clone in vivo have been established in vitro during the last decade (for recent reviews see Minowada et al. 1980a, Nilsson 1979). Lymphoid leukemia and lymphoma lines all seem to be arrested at a particular stage of lymphoid differentiation (Nilsson 1978). Attempts to induce differentiation with mitogens have generally been unsuccessful. Only in the promyelocytic leukemia cell line HL-60 has spontaneous terminal differentiation been documented (Collins et al. 1977; Gallagher et al. 1979). Recently, however, inducible differentiation in vitro has been demonstrated in three non-lymphoid hematopoietic cell lines using substances known to induce differentiation in animal cell systems. Thus, dimethyl-sulfoxide (DMSO) and 12-te-tradecanoyl-phorbol-13-acetate (TPA) will differentiate HL-60 cells (Collins et al. 1978; Gahmberg et al. 1979; Huberman and Callaham 1979: Rovera et al. 1979), butyric acid or hemin will induce the erythroleukemic K-562 cells (Andersson et al. 1979: Rutherford et al. 1979), and TPA or supernatant from mixed lymphocyte cultures (MLC), the histiocytic lymphoma U-937 cells (Koren et al. 1979; Nilsson et al. 1980a).