Pharmaceutical drugs, hazardous chemical compounds and other products must be evaluated for their hematological toxic potential for their safety use. In vitro CFU-GM clonogenic assay is one of the best test systems to evaluate the adverse toxic effects of xenobiotics on the blood forming system. The test involves exposing the mice bone marrow cells to various test substances at different concentrations and estimating the inhibition of the proliferation of the CFU-GM colonies. This technique is best use for determining the human maximum tolerated dose (MTD). It describes the use of in vitro CFU-GM assay for characterizing and predicting hazards to humans.