Adult bovine aortic endothelial (ABAE) cells specifically bound 125I-labelled high density lipoprotein3 (125I-HDL3) and saturation of the binding was observed at a concentration of about 50 to 100 μg protein/ml. The binding of purified apolipoprotein (apo) A-I and A-IV to cultured ABAE cells and the correlation of this binding with the binding of HDL to the cells was studied. ABAE cells bound 125I-labelled apo A-I (125I-apo A-I) and after a 3-hour exposure to the ligand, most of the radioactivity (82%) was associated with the cell surface and was accessible for trypsinization. Saturation of 125I-apo A-I binding was observed at a concentration of 3 μg/ml; each cell possessed 1.38 x 105 high affinity binding sites (Kd = 2.04 x 10-8 M). The cultures also bound 125I-labelled apo A-IV (125I-apo A-IV), and saturation of the binding was observed at a concentration of 2 μg/ml. Each cell possessed 5.4 x 104 high affinity binding sites for apo A-IV (Kd = 1.45 x 10-8 M). Addition of either excess unlabelled apo A-I or excess unlabelled apo A-IV competed with the binding of both iodinated apo A-I and apo A-IV. It suggests that apo A-I and A-IV share the same binding site on endothelial cells. HDL also successfully competed with the binding of apo A-I and A-IV to ABAE cells. However, the excess unlabelled HDL was found to absorb the 125I-apo A-I but not the 125I-apo A-IV, which indicates a true competition only between HDL and the iodinated apo A-IV. The binding of both apo A-I and A-IV was increased 90-fold upon preincubation of the cultures with 25-hydroxycholesterol (25-HC) and by four-fold when preincubated with acetylated low density lipoprotein (acetylated LDL). These results demonstrate the specific interaction of purified apo A-I and A-IV with ABAE cells probably via the same binding site and may indicate their role in the binding of HDL to endothelial cells.