Atherosclerotic lesions are hypoxic (Bjornheden et al. 1999) and have a low extracellular pH (Naghavi et al. 2002) (probably in coexistence) compared to the normal intima of an artery. Oxidised low density lipoprotein (LDL) is believed to be involved in atherosclerosis and apoptosis of macrophages occurs in atherosclerotic lesions. We have therefore investigated the effects of pH and hypoxia on apoptosis induced by oxidised LDL in macrophages. Human LDL was oxidised by dialysis for 24 h against 10μM CuSO₄ at 37^oC and contained high levels of oxysterols and only low levels of lipid hydroperoxides (as they had largely decomposed). Murine J774 macrophages were cultured in the absence or presence of oxidised LDL (100μg protein/ml) at 3% oxygen (hypoxia) or 20% oxygen (normoxia for cell cultures) at pH 7.4 or 7.0 for 18 h. Flow cytometry was used to measure apoptosis (annexin V binding) and necrosis (propidium iodide uptake). Oxidised LDL was damaging for most cells, causing a large increase in apoptosis and secondary necrosis (apoptosis followed by necrosis) of the macrophages at pH 7.4 under normoxic or hypoxic conditions. Low extracellular pH had a marked protective effect against cell death induced by oxidised LDL (decreasing apoptosis plus secondary necrosis) under both oxygen levels (78.5 ± 6.0% at pH 7.4 vs. 44.8 ± 5.3% at pH 7.0 for normoxia; mean ± SEM, n = 17 individual experiments, P<0.001). The healthy cell population increased from 15.8 ± 3.7% at pH 7.4 to 52.2 ± 5.2% at pH 7.0 (P<0.001). Similar effects were observed when the extracellular pH was lowered under hypoxic conditions. We conclude that low extracellular pH in atherosclerotic lesions may protect macrophages against the cytotoxic effects of oxidised LDL under both normoxic and hypoxic conditions.