Reaction of bovine serum albumin (BSA) with malondialdehyde (MDA), a product of lipid oxidation, resulted in the modification of amino residues of the protein to produce three kinds of adducts in the protein molecules, aminopropenal (1), N, N'-disubstituted 1-amino-3-iminopropene and 4-methyl-1, 4-dihydropyridine-3, 5-dicarbaldehyde (3).Modified BSA, in which 39 out of the total of 60 amino residues were modified, showed effective binding to thioglycollate-induced mouse peritoneal macrophages.MDA-modified BSA inhibited the binding of formaldehyde-modified BSA to the macrophages, indicating that MDA-modified BSA binds to the scavenger receptor for formaldehyde-modified BSA.However, the converse was not the case, suggesting that MDAmodified BSA binds to additional receptors to which formaldehyde-modified BSA does not. Reduction of the double bonds of 1 and 2, and the aldehyde functions of 1 and 3 in MDA-modified BSA did not affect the binding of the protein.However, modification of the aldehyde function of1 with glycine resulted in loss of the ligand activity of the protein.These results suggest that adducts 1, 2and3 in the BSA molecule are not directly involved in the binding to the scavenger receptor of macrophages, though adduct 1 may be located near the binding site or may play a role in maintaining the active conformation of the binding site.