The conversion of tissue macrophages into cholesteryl ester-rich foam cells is a crucial early event in atherogenesis. We studied the platelet as a potential source of cholesterol for esterification by macrophages because (a) platelets are rich in free cholesterol, (b) they adhere to macrophages early in atherogenesis, and (c) vascular injury can induce foam cell formation in the absence of hyperlipoproteinemia. We found that washed, activated human platelets from normocholesterolemic donors stimulated cholesteryl ester formation by the human monocyte-derived cell, U-937. Platelet cholesterol, released from platelets activated with calf skin collagen, was approximately equipotent at donating cholesterol to U-937 cells for esterification as normal human low density lipoprotein cholesterol. The stimulation of cholesteryl ester formation by activated human platelets demonstrated both concentration and time dependence. When hypercholesterolemic donors were studied, it was found that increasing plasma levels of cholesterol correlated directly with the ability of these hypercholesterolemic platelets to support cholesteryl ester synthesis by U-937 cells. Cholesterol-donating activity was also found in a 1,000-g supernatants of activated platelets. These observations point to a new and potentially important role for platelets in atherogenesis and suggest a mechanism for foam cell formation in the absence of marked hypercholesterolemia.