Three human monocytic cell lines, U-937, THP-1 and Mono Mac 6 have, because of their morphology and staining properties, been classed as cell lines frozen in a window of the monocyte differentiation lineage corresponding to monoblasts and/or immature monocytes. These cell lines were analyzed for expression of a panel of hematopoietic differentiation markers by Northern blot analysis. They were all found to express one or several biochemical markers characteristic of immature cells in monocytic development, including myeloperoxidase, N-elastase, cathepsin G, myeloblastin, and azurocidin. Normal peripheral blood monocytes did not express these markers. Moreover, several markers expressed at high levels in mature monocytes, such as lysozyme, CD14, MHC class II and alpha-1 antitrypsin were either not expressed or were expressed only at low levels in the three cell lines analyzed. These results show that arrested differentiation at a relatively early stage of monoblast development is a common denominator for these human monocytic cell lines. Thus, transforming mutations acting at such an immature differentiation stage may frequently lead to neoplastic transformation, whereas similar mutations occurring at a more mature differentiation stage never give rise to any leukemias due to the loss of proliferative potential in committed cells.