A cell line designated SKM-1 was newly established from leukaemic cells of a 76-year-old Japanese male patient with monoblastic leukaemia following myelodysplastic syndrome (MDS). The cells were obtained from peripheral blood of the patient when he lost multiple point mutations of ras genes with acquisition of chromosomal abnormalities during disease progression in MDS. The cells grew as a single floating cell, and have been continuously growing with the morphological characteristics of immature monoblasts by serial passages during the past 42 months with a doubling time of about 48 h. By cytochemical analysis, the cloned cells were positive for butyrate esterase, but negative for the Epstein-Barr virus associated nuclear antigen. Phenotypic analysis revealed the expression of myelomonocyte specific antigens such as CD4, CD13, CD33 and HLA-DR. Cells from the primary peripheral blood and those from 50 passages of the SKM-1 cell line both possessed no activated ras genes but showed karyotype abnormalities with 46,XY, del(9)(q13;q22), der(17) t(17;?)(p13;?). The SKM-1 cells have two mutations in p53 gene and overexpress the p53 products. This cell line may contribute to a better understanding of molecular mechanisms in the progression from MDS to myelogenous leukaemia.