We studied the importance of changes in plasma protein concentrations in patients with hematologic systemic diseases treated by selective decontamination of the digestive tract. The concentrations of albumin, acute-phase proteins, immunoglobulins and fibronectin were determined by laser nephelometry in 125 serum samples of eight patients. Results obtained allow the following conclusions: 1. There was a correlation between the clinical condition and concentrations of acid alpha-1-glycoprotein, IgG, CRP and fibronectin. These concentrations might be important for assessment of the course of the disease, effectiveness of therapy and the detection of pathogenetic relations. 2. The acid alpha-1-glycoprotein is of particular importance since relations exist between changes in its concentration and therapeutic results as well as the prognosis of the disease.