A specific,sensitive and simple radioligand binding assay for apoCⅢ-binding sites of hepatic plasma membranes has been established by separation of B/F with PEG.Addition of increasing concentration of ¹²⁵Ⅰ-labeled apoCⅢ to human hepatic plasma membranes revealed saturation binding to membranes with a K_d of 0.31μmol/L(3.1×10^-7mol/L) and binding maximum of 1.74μg/mg of membrane protein.In displacement studies using unlabeled apoCⅢ and isolated lipoproteins HDL,LDL and VLDL,only apoCⅢ and VLDL effectively competed with ¹²⁵Ⅰ-apoCⅢ for membrane binding sites. The binding of ¹²⁵Ⅰ-apoC l to human liver plasma membranes was Ca²⁺-independent and was abolished when plasma membranes were treated with trypsin. The characteristics of apoC Ⅲ-binding sites of mouse liver plasma membranes was similar to that of human liver plasma membranes with an exception of binding maximum of 1.52μg/mg of membrane protein.