The impact of plasmapheresis (PA) and immunosorption (IS) of low density lipoproteins (LDLP) on platelets was examined in patients with familial hypercholesterolemia. PA and IS sessions resulted in a decrease of platelet counts, aggregation activity in relation to TXA2 analogue, U46619, and capacity for adhesion and spreading over type-4 collagen-coated surface. All effects were of similar markedness in both procedures, i.e. they were unrelated to PA or IS specificity, but rather were due to platelet interaction with extracorporeal circulation circuit. Platelet changes seen immediately after the procedure were transitory. Platelet counts and capacity for aggregation and adhesion were recovered by the time of the next procedure (1 or 2 weeks later). Long-term (more than 6 months') use of PA or IS did not essentially affect platelet counts, aggregation and adhesion, but rather undermined platelet spreading capacity.