The study of mutant mice with altered or deficient hematopoietic or hemostatic gene products provides a challenge to the researcher, particularly when genetic alterations lead to lethal phenotypes. The following review provides a framework for understanding murine hematopoiesis, based on work with mutant mice, and details experimental approaches used to evaluate these animals. Mice with deficiencies in hemostatic and fibrinolytic system proteins are discussed, and the investigation of their phenotypes is reviewed.