The authors have evaluated the behaviour of protein C activity, factor X and factor VII coagulant activity, and serum lipoprotein(a) before and after haemodialytic treatment in the plasma of patients on maintenance haemodialysis. The plasma level of protein C activity, depressed before haemodialysis, significantly increased during the course of haemodialysis; factor X and factor VII increased as well despite heparinization; serum lipoprotein(a) was abnormally elevated before haemodialysis and did not change after haemodialysis. In vitro incubation (30' at 37 degrees C) of uremic and healthy blood samples resulted in a decrease of serum lipoprotein(a) concentration. After heparin addition (final concentration 0.5 U/ml) lipoprotein(a) became higher in uremic blood only.