The influence of low density lipoprotein (LDL) on platelet aggregability was studied using filtragometry and conventional Born aggregometry in vitro. Three different concentrations of autologous LDL, obtained from 9 healthy male volunteers, were incubated for 20 min, at 37°C, with whole blood anticoagulated with low molecular weight heparin (filtragometry) or citrated platelet-rich plasma (PRP; Born aggregometry). The LDL-cholesterol concentration was increased from 1.7 ± 0.2 mmol/1 to 2.4 ± 0.2, 3.5 ± 0.3 and 5.3 ± 0.5 mmol/l, respectively. Adenosine diphospate (ADP)-induced platelet aggregation in whole blood was enhanced in a dose dependent manner by LDL, as assessed by filtragometry (ADP cone, range 0.1-0.3 μM). Platelet aggregability in PRP (Born) was not affected by LDL at the ED(50) for ADP-induced platelet aggregation (i.e. 1-4 μM ADP). The marked platelet activation caused by the high ADP concentrations used with conventional Born aggregometry may have masked a modest LDL-induced platelet activation as a slight increase in spontaneous platelet aggregation was observed in PRP at the intermediate LDL-concentration. The present findings indicate that low concentrations of LDL stimulate platelet aggregability in the physiological whole blood milieu. This adverse effect of LDL-cholesterol may be of clinical importance in patients with hypercholesterolaemia.