As bone marrow transplantation becomes one of the preferred treatments for life-threatening diseases of the blood, increasing numbers of patients must seek compatible marrow from unrelated donors. The low probability of finding a good patient-donor marrow match means that between one- to two-thirds of ill patients never locate a donor with perfectly compatible marrow. Furthermore, once volunteers are identified as a preliminary match for a patient, 30 to 40 percent are either unwilling or medically unable to actually donate marrow. Thus, volunteer attrition contributes significantly to the problems already inherent in finding a suitable donor candidate.