Retrospective studies have provided indirect evidence that allogeneic blood transfusion may adversely influence the prognosis of cancer patients. This effect may be prevented by using autologous blood transfusions. However, this involves preoperative donation of blood, the consequences of which are still unknown. The aim of the present study was to investigate the possible effects of blood loss on tumour growth and on NK-cell activity. An artificial lung metastasis model was used in the BN rat from which 20 per cent of the blood volume was taken at different time intervals. The results showed that blood loss, one day prior to tumour challenge, had a profound stimulating effect on tumour growth. After blood loss, the number of lung metastases was doubled as compared to controls. This tumour-promoting effect could be prevented by an immediate plasma transfusion, but not by evoking a normal haemoglobin level after blood loss by pretreatment with recombinant erythropoietin (rEpo). The NK-cell activity of spleen cells was significantly depressed, 24 hours after blood loss. At a 50:1-lymphocyte-to-target cell ratio, the NK-cell activity dropped from 25.3 per cent in controls to 9.3 per cent in experimental animals. Since NK-cells are assumed to play a role in the clearance of tumour cells from the circulation, the enhanced tumour growth observed after blood loss might be caused by this depression.