A novel long-term cultured myeloid cell line was established from the bone marrow of a patient with myelodysplastic syndrome (MDS). This cell line, designated MDS92, proliferated in the presence of interleukin-3 or granulocyte-macrophage colony-stimulating factor and transiently in the presence of Steel factor, with a tendency for gradual maturation, and formed myeloid colonies in the semi-solid culture condition. In addition, the MDS92 cell line represented rather complicated karyotypic abnormalities including the deletion of fifth and seventh chromosomes and a point mutation at codon 12 of the N-ras oncogene. These characteristics of the MDS92 cell line are exclusively compatible with the property of preleukaemia.