To investigate the role of activated T lymphocytes in the formation of atherosclerotic lesions, we studied the influence of FK506, an immunosuppressant, on the development of atherosclerosis in cholesterol-fed rabbits. New Zealand White rabbits fed on a 1.5% cholesterol diet were administered FK506 at 0.05 mg/kg (n = 12), 0.1 mg/kg (n = 12) or isotonic saline (as the control, n = 12) intramuscularly three times a week for 12 weeks. Although FK506 treatment did not affect plasma lipid levels, it caused an increase in the development of atherosclerotic lesions in a dose-dependent manner. Immunohistochemical analysis of the aorta after 8 weeks on the diet revealed that the ratio of T lymphocytes to the total number of cells in the plaques decreased significantly in the FK506 treated rabbits compared to the control rabbits. In culture, FK506 did not affect smooth muscle cell proliferation and cholesteryl ester formation in the macrophages. In contrast, culture medium from lymphocytes stimulated by concanavalin A decreased the accumulation of cholesteryl ester in the macrophages. This effect was inhibited by the culture medium in the presence of FK506. These findings suggest that activated T lymphocytes may inhibit intracellular cholesterol accumulation in atherosclerotic plaque.