A monocytoid cell line (DOP-M1) was established from mononuclear cells separated from the cerebrospinal fluid of a 1-year-old girl with acute monoblastic leukemia (AMoL) (French-American-British; FAB-M5a). Judged by morphological, cytochemical, and immunological criteria, the DOP-M1 cells showed immature monocytoid characteristics. They were positive for alpha-naphthyl butyrate esterase, the expression of which was inhibited by NaF, and for myeloperoxidase (MPO). Positive MPO findings in nuclear envelope were detectable by electron microscopy. The cell surface was positive for CD15, CD33, and CDw65, but negative for CD4, CD14, and HLA-DR. HLA-DR expression was detected after treatment with IFN-gamma. Chromosome analysis of DOP-M1 cells revealed 47,X,-X,-13,+19,+20,+mar. Our established cell line, DOP-M1 appears to be a cell line which will be a useful tool for studying the phenotype, morphology, and function of monocytoid cells.