We characterized the uptake of fractionated [³H]heparin in rat peritoneal macrophages by investigating the influence of inhibitors of transport and by analyzing binding and internalization processes. Several ligands of scavenger receptors inhibited the binding and subsequent internalization. When temperature was lowered from 37°C to 4°C, the internalization was significantly reduced, while the binding was unchanged, as expected for receptor-mediated endocytosis. However, known inhibitors of receptor-mediated and adsorptive endocytosis of polypeptides and phagocytosis did not affect the uptake. These characteristics are similar to those observed in rat Kupffer cells in our previous study and suggest the involvement of scavenger receptors in the uptake of fractionated [³H]heparin. Finally, although the dissociation constant (K_d) in the macrophage was comparable with that in the Kupffer cell, the internalization rate constant (k_{int, app}) was approximately two-fold larger in the former case, suggesting that the functional regulations of scavenger receptors may differ between these cells.