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Three studies were conducted to determine whether the elevated plasma and HDL cholesterol levels observed in copper-deficient rats could be explained by the interaction of ¹²⁵I-HDL subfractions with liver membrane preparations in vitro. Rats from all studies were randomly divided into two dietary treatments, copper-deficient and adequate (0.7 mg and 8.0 mg Cu/kg diet, respectively). Deionized water and diet were provided ad libitum. After eight weeks, rats were exsanguinated, membranes prepared from livers, and plasma high density lipoproteins (HDL) isolated by ultracentrifugation and agarose column chromatography. Heparin-Sepharose affinity chromatography was used to isolate specific subfractions of HDL. The HDL subfractions derived from rats of each dietary treatment were iodinated and bound to either crude liver membranes or purified liver plasma membranes prepared from rats of both treatment groups. Total binding data and computer derived estimates (K(d) and B(max)) were used to compare differences between treatments. Binding data from all experiments conformed to a one-site model. In all cases, binding was saturable and EDTA and pronase insensitive. Treatment differences were observed in Study I (¹²⁵I-apo E-free HDL binding to crude liver membranes). Significantly lower total binding and B(max) were observed when lipoproteins and membranes from copper-deficient animals were used in the assay. Competition experiments from Studies II and III demonstrate that the different HDL subfractions competed effectively with one another for binding sites, indicating that apo E is not a determinant in binding of rat ¹²⁵I-HDL subfractions to purified liver plasma membranes.