A ll classes of lipoproteins considered to be atherogenic contain apo-B100 or apo-B48. However, there is a distinct paucity of data regarding whether lipoproteins containing apo-B48 or apo-B100 differ in their intrinsic ability to promote the development of atherosclerosis. To address this issue, we compared the extent of atherosclerosis in three groups of animals: apo-E-deficient mice (apo-B+/+apo-E-/-) and apo-E-deficient mice that synthesize exclusively either apo-B48 (apo-B48/48apo-E-/-) or apo-B100 (apo-B100/100apo-E-/-). Mice (n = 25 in each group) were fed a chow diet for 200 days, and plasma lipid levels were assessed throughout the study. Compared with the levels in apo-B+/+apo-E-/- mice, the total plasma cholesterol levels were higher in the apo-B48/48apo-E-/- mice and were lower in the apo-B100/100apo-E-/- mice. However, the ranges of cholesterol levels in the three groups overlapped. Compared with those in the apo-B+/+apo-E-/- mice, atherosclerotic lesions were more extensive in the apo-B48/48apo-E-/- mice and less extensive in the apo-B100/100apo-E-/- mice. Once again, however, there was overlap among the three groups. The extent of atherosclerosis in each group of mice correlated significantly with plasma cholesterol levels. In mice from different groups that had similar cholesterol levels, the extent of atherosclerosis was quite similar. Thus, susceptibility to atherosclerosis was dependent on total cholesterol levels. Whether mice synthesized apo-B48 or apo-B100 did not appear to have an independent effect on susceptibility to atherosclerosis.