The effects of chylomicron remnants on lipid accumulation in J774 macrophages, and the incorporation of radioactivity from remnant lipids radiolabelled with [<sup>3</sup>H]oleate into cellular lipids was investigated. After 24 h of incubation with chylomicron remnants, there was considerable accumulation of lipid within the cells as assessed by staining with oil red O, indicating that the particles induce the formation of foam cells. Fatty acids released from the radiolabelled remnant lipids after uptake were found to be incorporated into cellular triacylglycerol (52%), phospholipid (37%) and cholesteryl ester (8%), but at higher remnant concentrations, the proportion used for triacylglycerol formation increased (up to 73%). When the macrophages were shifted into a pro-oxidising or pro-reducing state by incubation (24 h) with CuSO<sub>4</sub> (2.5 µ M) or N-acetylcysteine (5 m M), respectively, the incorporation of [<sup>3</sup>H]oleate from remnant lipid into cellular triacylglycerol and phospholipid was increased by 20–30% in the more oxidised as compared with the more reduced cells. These findings indicate that exposure of J774 macrophages to chylomicron remnants leads to the accumulation of lipid within the cells, and that this process is enhanced by pro-oxidising conditions. We conclude, therefore, that both lipids of dietary origin and the redox balance within macrophages may have a significant role in the induction of foam cell formation.