The change in the protein concentration in blood serum was investigated after implantation of a pacing system with epoxide surface (group A, 20 patients) or metallic case (group B, 9 patients). The response to surgical intervention was equal in both groups. In the first period following operation the acute phase reaction took place in terms of increasing concentrations of alpha-globulins, especially alpha 1-acid glycoprotein, alpha 1-antitrypsin, ceruloplasmin, haptoglobin, and beta 1A/C-globulin, Simultaneously, a decrease in prealbumin, albumin, beta-globulin, in particular haemopexin and transferin concentration appeared. As for immunoglobulins, a decrease in IgG and increase in IgA and IgM globulins was observed. In the course of a longer period of time following implantation (group B--three months; group A--two years) a decrease of almost all investigated proteins was observed with exception of increased IgA globulin and transferin in group A.