A po E, a key regulator of cholesterol-rich lipoprotein metabolism, is synthesized by numerous extrahepatic tissues. Although its synthesis in macrophages is documented, the contribution of macrophage-derived apo E to hepatic clearance of serum cholesterol is unknown. To address this issue bone marrow transplantation was performed on hypercholesterolemic apo E-deficient mice with either syngeneic apo E-deficient mouse bone marrow cells (E0-control) or wild-type mouse bone marrow cells expressing apo E (E0-treated). E0-control and E0-treated mice were fed either a regular chow diet or an atherogenic diet (designated E0-control-HF and E0-treated-HF). Serum cholesterol levels dropped dramatically in the E0-treated mice largely due to a reduction in their VLDL cholesterol. No changes were seen in the E0-control mice. After 4 wk serum cholesterol in E0-treated-HF mice was about four-fold lower compared to E0-control-HF animals. Moreover, the extent of atherosclerosis in the E0-treated-HF mice after 14-16 wk was greatly reduced. Wild-type apo E mRNA was detected in the liver, spleen, and brain of the E0-treated mice indicating that apo E gene transfer was successfully achieved through bone marrow transplantation. More importantly, the level of apo E expression was sufficient to reduce the severe hypercholesterolemia of the apo E-deficient mice fed either chow or atherogenic diets.