Donor deferral rates in regional blood centers vary from 5 to 24 per cent, reducing by more than 1,250,000 the number of units of volunteer blood available for transfusion in the nation each year. Those criteria for donor deferral which are intended to exclude donors likely to suffer a "donor reaction" are based partially on untested hypotheses and tradition. In a six-month prospective study, we adopted more liberal criteria for donor acceptance. During this period, donor reaction rates did not increase, and the deferral rate fell from 10 to 7 per cent. Our findings suggest that less restrictive criteria can be used for donor selection without compromising donor safety. If all blood centers reduced their deferral rates to 7 per cent, the nation's blood supply would be increased by more than 500,000 units annually.