The heparin-induced extracorporeal precipitation of low density lipoproteins (HELP) is a well established clinical apheresis procedure to markedly reduce cholesterol levels. The biocompatibility of this filter system was investigated by the determination of lipid peroxidation products. Both lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS) were determined before, during and after 38 aphereses in 21 patients undergoing regular HELP treatment. Although HELP patients had significantly elevated TBARS compared to 93 healthy controls (3.13 +/- 0.64 vs. 1.66 +/- 0.50 mumol L-1; P < 0.01), no significant differences were observed compared to either 104 patients suffering from angiographically confirmed coronary heart disease (3.42 +/- 0.81 mumol L-1; P > 0.05 vs. HELP patients) or 38 aged-matched hyperlipidaemic patients (3.30 +/- 0.75 mumol L-1; P > 0.05 vs. HELP patients), neither of which were included in the HELP programme. No lipid hydroperoxides were detected in the plasma of HELP patients either before or after the extracorporeal treatment. After the LDL-apheresis TBARS were significantly decreased (2.60 +/- 0.52 mumol L-1) compared to the values before the treatment (P < 0.01). There was no evidence for the formation of lipid hydroperoxides within the HELP circuit. It is suggested, therefore, that plasma lipids are not oxidized by the HELP procedure.