It has recently been shown that extrahepatic cells can eliminate intracellular cholesterol by enzymatic conversion into 27-hydroxy-cholesterol and 3 beta-hydroxy-5-cholestenoic acid. Using immunohistochemical methods, we studied the presence of the enzyme responsible for these conversions, sterol 27-hydroxylase, in human carotid atherosclerotic plaques. All plaques examined were found to contain sterol 27-hydroxylase immuno-reactive cells. While some endothelial cells stained for sterol 27-hydroxylase, the majority of the immunoreactive cells co-localized with macrophages. Accumulation of sterol 27-hydroxylase-positive cells were often observed in macrophage-rich core regions of complicated lesions. High concentrations of 27-hydroxycholesterol were found in plaques, while the concentration in non-atherosclerotic human vessels was lower by two orders of magnitude. The rabbit, which is particularly sensitive to dietary cholesterol and easily develops fatty streaks, had low plasma levels of 27-hydroxycholesterol, 3 ng/ml compared to 150 ng/ml in humans. The concentration of 27-hydroxycholesterol in the atherosclerotic rabbit vessels was also lower compared to human atherosclerotic plaques. The results are consistent with our hypothesis that sterol 27-hydroxylase may be utilized by human macrophages as a defence towards a high cholesterol load. This mechanism may be less important in some other species.