We investigated the effect of plasma low density lipoproteins (LDL), very low density lipoproteins (VLDL), and high density lipoproteins (HDL) on the platelet inhibitory effect of primary cultures of human endothelial cell monolayers (ECM). ECM incubated with lipoprotein-deficient plasma (LDP) for 2 hours at 37°C had an inhibitory effect on ADP- and collagen-induced platelet aggregation and prostaglandin production in platelet-rich plasma similar to that observed when ECM were preincubated with growth medium or plasma. Concentrations of LDL in LDP up to a protein concentration of 1600 μg/ml had an inhibitory effect on the endothelial cells' ability to modulate these platelet reactions. VLDL at the highest concentration (1600 μg/ml) had a slightly inhibitory effect, whereas HDL showed no such effect. The inhibitory effect of LDL was not observed during the first hour of incubation. When HDL in concentrations similar to or higher than LDL were combined with LDL, the inhibitory effect of LDL was partially reduced. VLDL combined with LDL or HDL did not interfere with the effects of the later fractions. The inhibitory effect of LDL was significantly reduced when LDL were diluted in whole plasma. Prostacyclin which is synthesized and released from the endothelial cells and contributes to the inhibitory effect upon platelets did not change its effect on platelet reactivity by preincubation with the various lipoprotein fractions. The current studies may indicate that LDL have a direct effect on the endothelial cells and that this effect may be partially counteracted by HDL. This effect of LDL on the endothelial cells reduces the endothelium's ability to inhibit platelet aggregation and thus could favor the tendency to thrombus formation.