In vitro models of hematopoiesis are used increasingly in investigative hematopathology. Such models complement in vivo animal testing and have been shown to be predictive for hematotoxicity associated with anticancer and antiviral agents in humans. In vitro models of hematopoiesis consist of short-term cloning assays for various hematopoietic progenitor cells and long-term functional assays for the marrow hematopoietic microenvironment. In our laboratories, the cloning assays have been used as investigative tools to study the pathogenetic mechanisms of drug-induced blood disorders and as screening systems to investigate the possible hematotoxic potential of candidate drugs in various animal species. Data in support of these applications are presented in this paper.