Blood transfusion and recurrence of colorectal cancer: the role of platelet derived growth factors. Efforts to explain the possible effects of blood transfusion on the recurrence of colorectal cancer have been based entirely on the immunosuppressive effects of blood transfusion. However, the relationship between solid tumour development and the immune system is inconclusive. We have investigated an alternative mechanism involving the potential role of growth factors in this phenomenon. Using a human fibroblast: [125I]deoxyuridine uptake mitogenesis assay, the relative amounts of growth factor in the plasma of stored blood were measured. There was a progressive increase in mitogenesis from day 0 (n = 6) to day 28 (n = 6; P less than 0.001, Mann-Whitney U test). The effect of growth factors on the development of liver and intraperitoneal metastases was studied in Hooded Lister rats. Following an intraportal injection of 10(5) MC28 tumour cells, the experimental group (n = 25) received 2 ml of syngeneic serum intravenously for 4 days. Likewise, colonic anastomoses were performed on omentectomized rats and the peritoneal cavity seeded with 10(3) cells. The experimental groups (n = 20) received either 2 ml serum intravenously repeatedly or 3 ml serum intraperitoneally (n = 19). There was no significant increase in liver metastases or peritoneal disease following intravenous infusion of serum but serum delivered intraperitoneally resulted in a significant increase in tumour from 22 per cent in the controls to 89 per cent in the study group (P less than 0.01). Growth factors released from platelets following blood loss into the peritoneal cavity may be important in enhancing local recurrence of colorectal cancer. 