Continuous human malignant hematopoietic (MH) cell lines have become invaluable tools for hematological diagnosis and research. Over the last 35 years several hundred cell lines spanning almost the whole spectrum of hematopoietic cell lineages have been described. The cardinal features of MH cell lines are their monoclonal origin, arrest of differentiation, genetic alterations, and unlimited proliferation; the major advantages of cell lines are the unlimited supply of cell material and the infinite storability and recoverability at will of the cells. Categorization of cell lines usually follows the physiological stages of hematopoietic differentiation in the various cell lineages. For an adequate classification, a detailed and comparative characterization of both primary and cultured cells is absolutely necessary. New cell lines, in particular, must be adequately characterized. While clinical and cell culture data and immunological and cytogenetic features are the most important data, cell lines should be described in as much additional detail as possible allowing any singular features to be pointed out. In addition to detailed characterization, immortality of the culture, proof of neoplasticity, authentication of the true origin of the cells, scientific significance and availability of the cell line for other investigators are of paramount importance. In summary, MH cell lines have the potential to greatly facilitate diverse studies of normal and malignant hematopoiesis; to that end, these cell lines must be extensively characterized and adequately described.