In the present work we have investigated the binding capacity, to cell membrane of bovine aortic endothelial cells (BAEC) cultured in vitro, of heparan sulfate (M(r) 11.1 kDa) and its four fractions named A (M(r) 24.5 kDa), B (M(r) 10.7 kDa), C (M(r) 9.1 kDa) and D (M(r) 6.8 kDa). Experiments were performed in comparison with unfractionated heparin (M(r) 12.9 kDa). The binding of the examined compounds was determined by the degree of displacement of [3H]-heparin from the cell membrane. The efficiency of heparan sulfate fractions B, C and D in displacing labelled heparin was found to decrease in function with the decrease of the molecular masses of the compounds. [3H]-heparin could be efficiently displaced mainly by heparin and fraction A, the fraction which has the highest molecular weight and represents only 3.8% of the unfractionated HA 90681. These experimental results may confirm that molecular mass is an important feature for the interaction of glycosaminoglycans with the specific binding sites on the cell surface.