M acrophage- and smooth muscle cell (SMC)-derived foam cells are typical constituents of human atherosclerotic lesions. At least three receptor systems have been characterized that could be involved in the development of foam cells: alpha 2-macroglobulin receptor/LDL receptor-related protein (alpha 2 MR/LRP), scavenger receptor, and LDL receptor. We studied the expression of these receptors in human atherosclerotic lesions with in situ hybridization and immunocytochemistry. An abundant expression of alpha 2MR/LRP mRNA and protein was found in SMC and macrophages in both early and advanced lesions in human aortas. alpha 2MR/LRP was also present in SMC in normal aortas. Scavenger receptor mRNA and protein were expressed in lesion macrophages but no expression was found in lesion SMC. LDL receptor was absent from the lesion area but was expressed in some aortas in medial SMC located near the adventitial border. The results demonstrate that (a) alpha 2MR/LRP is, so far, the only lipoprotein receptor expressed in lesions SMC in vivo; (b) scavenger receptors are expressed only in lesion macrophages; and (c) both receptors may play important roles in the development of human atherosclerotic lesions.