Atherosclerosis is a major cause of morbidity and mortality in developed countries. In humans the risk of atherosclerosis is inversely correlated with plasma levels of high density lipoprotein (HDL). As a step in determining whether the experimental reduction of plasma HDL level will increase susceptibility to atherosclerosis, we have used gene targeting in embryonic stem cells to produce mice lacking apolipoprotein A-I, the major protein component of HDL particles. Mice homozygous for the disrupted gene have no plasma apolipoprotein A-I detectable by double immunodiffusion; their total plasma cholesterol and HDL-cholesterol levels after overnight fasting are reduced to about one-third and one-fifth of normal levels, and they are grossly deficient in alpha-migrating HDL particles.