In recent years it has been suggested that fibronectin is a non-specific opsonic protein of significance in host resistance against septic complications following trauma and surgery. The aim of the present study was to investigate the fluctuations in serum levels of fibronectin, immunoglobulins IgG and IgM, and complement factors C3 and C4 postoperatively. Serum samples were obtained pre-operatively and at different intervals postoperatively in 72 patients admitted for elective surgical procedures on the alimentary tract, the heart and the hip joint. The mean pre-operative fibronectin values were 257 +/- 79 micrograms/ml. The lowest values (166 +/- 65 micrograms/ml) were found immediately after the operation, followed by an increase which did not reach pre-operative levels within 120 h. Five patients who developed postoperative complications and 14 patients with malignant disease did not differ significantly either in their pre- or postoperative fibronectin levels, compared with the whole group. The lowest fibronectin levels seen were also significantly above the concentrations necessary to promote binding of gelatinized test particles to macrophages and monocytes in vitro. There was a significant decrease in C3, C4 and IgG levels postoperatively whereas IgM levels were unaltered. The observed reduction did not extend below the lower normal levels for these proteins. The probable mechanism behind the reduction in serum levels seems to be a sequestration at the site of tissue injury. The postoperative reduction in circulating fibronectin levels has probably no implications for host resistance against septic complications. When sequestered into the site of injury, it may, however, be of significance for normal wound healing, due to its cell-matrix interactions.