The need for allogeneic blood can be reduced effectively with the use of preoperatively donated autologous blood. However, this collection also results in the waste of autologous blood in a considerable number of patients. In order to reduce overcollection, we requested that the amount of autologous blood donated before a total hip replacement be decreased from four units to two units. We then compared the amount of allogeneic blood required for fifty patients who had donated two units of autologous blood with that of fifty historical controls who had donated four units. The patients were matched for sex, initial hemoglobin concentration, and blood loss. There was no significant difference in the percentage of autologous donors who received additional allogeneic blood; 20 per cent (ten) of the patients who had donated two units and 16 per cent (eight) of the patients who had donated four units required allogeneic blood (p = 0.12). Eighteen autologous units from the patients who had donated two units and fifty-one units from the patients who had donated four units were discarded. The erythropoietic response, measured by the increase in the reticulocyte count, was comparable for the two groups. Iron stores, measured as the serum ferritin concentration, were lower after the drawing of four units (33 +/- 39 micrograms per liter) compared with after the drawing of two units (49 +/- 29 micrograms per liter) (p = 0.03). The donation of two units of blood does not increase the exposure to allogeneic blood compared with the exposure after the donation of four units by patients who are going to have a total hip replacement.(ABSTRACT TRUNCATED AT 250 WORDS)