The human apoSAA proteins comprise both acute phase (apoSAA1, apoSAA2) and constitutive (apoSAA4) isoforms; all are expressed in human atherosclerotic lesions as well as in liver. Recombinant acute phase apoSAA binds cholesterol with an affinity of approximately 170 nM and enhances cholesterol uptake by HepG2 cells (J. Lipid Res. 1995. 36:37-46). In the present study, we sought to define the region of acute phase apoSAA involved in cholesterol binding and to investigate the ability of constitutive apoSAA4 to bind cholesterol. Binding of [3H]cholesterol to apoSAAp was inhibited by unlabeled cholesterol (1-100 nM), but not significantly by vitamin D and estradiol. Direct binding of acute phase, but not constitutive, apoSAA to the surfaces of polystyrene microtiter wells was strongly diminished in the presence of cholesterol. The ability of apoSAAp to bind cholesterol was inhibited by antibodies to human apoSAA1 and to peptide 1-18 of apoSAA1. There was only slight inhibition of cholesterol binding by antibodies to peptide 40-63, and no inhibition by antibodies to peptides spanning regions containing amino acid residues 14-44 and 59-104. [3H]cholesterol uptake by neonatal rabbit aortic smooth muscle and HepG2 cells was enhanced by a synthetic peptide corresponding to amino acids 1-18 of hSAA1, but not by peptides corresponding to amino acids 1-18 of hSAA4. [3H]cholesterol uptake by HepG2 cells was slightly increased by a peptide corresponding to amino acids 40-63 of hSAA1. These findings suggest that apoSAA modulates the local flux of cholesterol between cells and lipoproteins during inflammation and atherosclerosis.