The immunoreactivity of cancer patients submitted to surgery and perioperatively transfused was investigated. Peripheral blood mononuclear cells (PBMC) and tumor infiltrating lymphocytes (TIL) were tested for the natural killer (NK) cytotoxic activity, for the in vitro synthesis of interleukin-2 (IL-2), interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and prostaglandin E2 (PGE2). The serum levels and the production of PGE2 by PBMC were significantly higher in patients than in controls, whereas no significant differences in the tested immunological variables emerged between the two groups of subjects. Instead, TIL produced significant larger amounts of spontaneous PGE2 (p < 0.001) and significant lower amounts of IFN-gamma (p < 0.001) and TNF-alpha (p < 0.001) than autologous PBMC, suggesting an involvement of PGE2 in the impairment of the host immunoreactivity at the tumor site. To evaluate the immunomodulating effect of blood transfusion, the patients were reexamined 8 to 20 days after surgery. No differences were found in the NK cytotoxic activity, lymphokine synthesis, serum levels, and production of PGE2 between transfused and untransfused patients. These results do not support the hypothesis that blood transfusions negatively affect the immune response of neoplastic patients.