During haemodialysis in patients with chronic renal failure plasma proteins have contact with dialysis membranes. Complement activation is one of effects of this contact. Intensity of this activation depends on structure of material that the membrane is made of, and it is a determinant of hemocompatibility of dialysis membrane. Our studies were designed to evaluate complement activation during haemodialysis in patients with chronic renal failure and therefore measurements of concentration of C3c factor in serum of hemodialyzed patients were performed. Furthermore, changes in concentration of C3c factor in serum during first and second use of cuprophan and polysulfone dialysis membrane were compared. An artificial kidney Fresenius 4008E, polysulfone dialyzers F5, cuprophan dialyzers C121 and acetate dialysing solution were used in the study. For the purpose of our study we have selected 10 patients hemodialyzed three times a week for 4 hours (5 of them suffered from diabetic nephropathy and the rest of our patients had chronic pyelonephritis) and 10 healthy subjects. Serum concentration of C3c was measured before haemodialysis, in 15', 60' and 240' minute of haemodialysis; measurement were performed on first and second use (after reutilization) of polysulfone and cuprophan membranes, respectively. C3c concentration in blood samples was measured with special reagent kits (turbidometric method). Statistically significant increase of serum C3c level was observed in studied group of patients in 15th minute of first use of cuprophan dialysing membrane. When values of C3c concentration in patients' serum during 60' and 15' of haemodialysis on first use of cuprophan membrane were compared, it was shown that there was a statistically significant decrease of serum C3c level in 60th minute of haemodialysis. 1. Haemodialysis in patients with chronic renal failure is associated with complement activation via the alternative pathway. 2. Maximal activation of complement pathway takes place in 15th minute of the first haemodialysis with use of cuprophan membrane. 3. Usage of the polysulfone dialyzer has no influence on concentration of C3c factor in serum during haemodialysis 4. Polysulfone membranes are characterized with higher haemocompatibility than cuprophan membranes.