Systems used to study the nature of the leukemic cell and predict treatment outcome in patients with myeloproliferative disorders. Cell culture techniques have been used to study normal and leukemic hematopoiesis. Investigations provided data on the nature of the clonogenic leukemic cell, its patterns of in vitro growth, role of stimulating factors, and effects of chemotherapeutic agents. Assay systems developed to study leukemic progenitor cells or self-renewal capacity have been useful clinically as predictive indicators of success of remission-induction therapy and have allowed testing of drug sensitivity in vitro. Alternative approaches using short- and long-term suspension cultures have provided systems to evaluate leukemic cells, including nonclonogenic, biochemical, and kinetic events. In vitro systems have been introduced to measure cell cycle events accurately and to identify metabolic mechanisms of drug resistance. The information provided by all of these systems has allowed a more comprehensive and multifocal characterization of the nature of the leukemic process and has better directed our approach to treatment. 