The effect of hypercholesterolemia on the platelet surface charge was examined in rabbits fed a lipid-rich diet (0.5% cholesterol and 5% butter). The strong anionic sites were detected with cationized ferritin (CF) pI 8.4, and the sialic acid concentration was evaluated by biochemical assays. In normal rabbits (average plasma cholesterol 0.36 +/- 0.05 mg/ml, and total platelet sialic acid 30.03 +/- 6 micrograms/mg protein) the platelet surface displayed a homogeneous distribution of CF, which also labeled the open canalicular system. Beginning with the third week of diet, at a plasma cholesterol level of 4.6 +/- 0.3 mg/ml, a reduction in the overall platelet negative charge was observed. As the diet progressed and the plasma cholesterol level increased, the CF binding to platelet surface diminished up to an almost total disappearance when the plasma cholesterol reached 18 mg/ml (the 20th week of diet). At the same time a progressive decrease in the sialic acid content up to 5.1 micrograms/mg protein was detected. These results suggest that diet-induced hyperlipidemia causes significant alterations in the platelet surface negative charge, especially in the sialic acid content.