The effects of blood transfusion, blood components, and surgical trauma on the growth of an experimental sarcoma have been examined. Recipient animals were inbred adult female WAB rats, which received allogeneic transfusions from inbred adult female PVG rats, syngeneic blood from inbred colony-mates, or saline infusions. Small volume transfusions (1-4 ml) of whole blood had no effect on tumour growth, but growth of the MC7 sarcoma was significantly enhanced following allogeneic transfusions of 5 ml whole blood, or when 4 ml was combined with sham laparotomy. Maximal enhancement of tumour growth occurred when 4 ml transfusions of allogeneic washed cells were given, but allogeneic plasma was also able, to a lesser degree, to enhance tumour growth. These data confirm that blood transfusion may enhance growth of the MC7 sarcoma, that the effect may be dose dependent, and synergistic with the immunosuppression of surgery. Many components of an allogeneic transfusion may be responsible for this effect.