In this unit, protocols to culture human platelet progenitors (Colony Forming Unit-Megakaryocyte; CFU-M) with or without toxicants are described. Platelet progenitors are obtained from human umbilical cord blood. After separation of mononuclear cells, the cell suspension can be cryopreserved or plated immediately. Megakaryocytes are identified by immunocytochemistry. Test chemicals are added to the culture medium before cells are plated. Megakaryocytes are scored after 12 days of culture. IC(10), IC(50) and IC(90) can be calculated by comparison to control cultures. A predictive model is proposed to evaluate the hazard of thrombocytopenia induced by chemicals. When IC(50) and IC(90) are below C(max) in humans, the likelihood of thrombocytopenia is strong.