—The present study examines the expression of ATP-binding cassette transporter 1 (ABC1) mRNA in normal and atherosclerotic tissues by using in situ hybridization in an effort to better understand the function of this cholesterol transport protein. Samples of normal baboon tissues as well as human normal and atherosclerotic aortas were hybridized with ³⁵S-labeled ABC1 sense and antisense riboprobes. Widespread expression of ABC1 was observed generally in tissues containing inflammatory cells and lymphocytes. Other noninflammatory cells that were also sites of ABC1 synthesis included the ductal cells of the kidney medulla, Leydig cells in the testis, and glial cells in the baboon cerebellum. Although normal veins and arteries did not express ABC1 mRNA, it was found to be upregulated in the setting of atherosclerosis, where widespread expression was found in macrophages within atherosclerotic lesions. These results are consistent with the proposed role of ABC1 in cholesterol transport in inflammatory cells. The specific upregulation of ABC1 mRNA in the setting of atherosclerosis probably reflects the response of leukocytes to cholesterol loading. However, the presence of ABC1 in ductal cells of the kidney medulla and in the small intestine suggest a more general role for this protein in cholesterol transport in other cell types.