The aim of this study was to assess the changes affecting natural killer cytotoxic cell (NKCC) activity following intraperitoneal implantation of a double veloured polyester prosthesis in a rat model. Blood samples were taken by cardiac puncture 1 h before (base line) and 14, 28, 100 and and 180 days post-implantation. Peripheral blood mononuclear cells were separated from heparinized blood by density centrifugation. A standard, 4 h 51Cr-release assay against YAC-1 target cells at effector to target ratios of 12:1; 25:1 and 50:1 was performed and the number of total leukocytes, lymphocytes, granulocytes, monocytes, and large granular lymphocytes (LGLs), as well as serum corticosterone levels (radioimmunoassay method) were determined. Comparative analysis of the results obtained from animals with implants, baseline samples, and a control group (laparotomy only) revealed lower NKCC, LGL, leukocyte and lymphocyte counts and elevated plasma corticosterone levels in animals receiving the implant on the 14th day post-implantation. Our findings indicate that the polyester implant can transiently modulate immune system activities. Since NK cells are important in the control of viral infection and carcinogenesis in humans, it is possible that the stress generated by polyester prostheses can exacerbate the surgical stress and put patients at a higher risk for viral infection and/or metastases.