Oxidation of LDL is now widely accepted to be involved in atherogenesis. The aim of this study was to examine the effect of BO-653, a strong radical scavenger and antioxidant, on oxidation of LDL by human macrophages in vitro. Fifty microg/ml LDL protein was incubated with macrophages in Ham's F10 medium, supplemented with additional Fe2+, for up to 48 h. Then the medium was analysed by LDL agarose gel electrophoresis, the thiobarbituric acid assay and gas chromatography. In the absence of added exogenous antioxidants, after 24h LDL oxidation produced 30.48 nmoles MDA equivalents/mg LDL protein and a relative electrophoretic mobility of 4.74. Linoleic acid (18:2), arachidonic acid (20:4) and cholesterol were depleted and 7beta-hydroxycholesterol was generated. BO-653 completely inhibited this cell-mediated oxidation of LDL in concentrations as low as 5 microM, being more effective than either alpha-tocopherol or probucol, which completely inhibited oxidation at 200 and 80 microM and only partially at 80 and 8 microM, respectively. This inhibition of cell-mediated LDL oxidation was not due to toxicity, as alpha-tocopherol, probucol and BO-653 were not toxic for the macrophages at the concentrations tested. Eighty microM alpha-tocopherol, 8 microM probucol and 5 microM BO-653 significantly reduced the toxicity to the oxidising culture caused by LDL oxidation. The results show that in this system BO-653 is a more effective antioxidant than alpha-tocopherol or probucol.