Plasma fibronectin is regarded to play an important part in a decrease of the resistance to infections. To specify the role of fibronectin in the pathogenesis of infectious complications in patients with depressions of hemopoiesis, the content of this opsonin was measured by ELISA in 113 patients with different patterns of hemoblastoses, lymphoproliferative diseases and with an aplastic syndrome. In 42 patients, the concentration of opsonin was measured in the presence of the superimposed infection of varying gravity. The fibronectin content was examined in 39 patients before, during and after completion of the cytostatic polychemotherapy. It turned out that in patients with paraproteinemic hemoblastoses, lymphogranulomatosis, aplastic anemia, chronic lympholeukemia, acute lympho- and myelo(mono)blastic leukemias, cyclic neutropenia, chronic myelosis and hematosarcomas, the concentration of fibronectin remained normal in the absence of infections. The computation of the linear correlation ratio did not reveal any association between the opsonin level and the concentration of neoplastic elements in the peripheral blood. Repeated measurements of the fibronectin level in patients whose underlying disease ran its course in association with marked neoplastic fever failed to detect any deficiency of the glycoprotein. The lowering of the fibronectin level was recorded in patients with a grave concomitant infection of the type of sepsis, necrotic enteropathy and lobar pneumonia. The degree of opsonin deficiency correlated with the patients' disease gravity. Prolonged reduction in the blood fibronectin level was of unfavourable prognostic importance. Cytostatic polychemotherapy, myelotoxic agranulocytosis as well as infectious complications of low gravity did not influence the concentration of fibronectin.(ABSTRACT TRUNCATED AT 250 WORDS)