Hematopoietic cell lines phenotypically and genotypically corresponding to original patient's leukemic or lymphoma cells are the powerful tools for studies of many aspects of malignant process and also for studies of normal and malignant hematopoesis. Until recently, permanent cell lines were derived from nearly all types of hematopoietic malignancies--leukemias and lymphomas. Availability of leukemia and lymphoma cell lines enabled the development of hybridoma technology for production of monoclonal antibodies and also discovery of the first human retroviruses HTLV-1,2 and HIV. Last but not least, leukemic cell lines are also used as standards for molecular diagnostics of leukemias and lymphomas. Leukemic and lymphoma cell lines have fully proved and extended their importance for modeling of malignant process ex vivo.