Heparinase depolymerized low molecular weight (LMW) heparin (Tinzaparin sodium, Logiparin®) was radiolabelled by catalytic tritiation to high specific radioactivity and the binding to fetal bovine heart endothelial (FBHE) cells was studied at 4°C and 37°C. The binding was found to be time dependent and saturable. Two classes of binding sites could be distinguished from Scatchard analysis at both temperatures: One with high affinity (K_D = 0.027 μM at 4°C, K_D = 0.012 μM at 37°C) and another with very low affinity (K_D = 69 μM at 4°C and 37 μM at 37°C). The binding reversibility was affected by the temperature indicating internalization of a fraction of the bound LMW heparin. At 4°C only 11% of the specifically bound heparin was bound irreversibly. At 37°C the non displaceable fraction accounted for 28 % of the specifically bound LMW heparin. This work demonstrates that tinzaparin sodium binds specifically to endothelial cells. This binding may be useful in interpreting pharmacokinetic properties of this low molecular weight heparin.