Human (Hu) lecithin-cholesterol acyltransferase (LCAT) is a key enzyme in the plasma metabolism of cholesterol. To assess the effects of increased plasma levels of LCAT, four lines of transgenic mice were created expressing a Hu LCAT gene driven by either its natural or the mouse albumin enhancer promoter. Plasma LCAT activity increased from 1.2- to 1.6-fold higher than that found in control mouse plasma. Lipid profiles, upon comparing Hu LCAT transgenics to control animals, revealed a 20 t0 60% increase in total and cholesteryl esters that were mainly present in HDL. The in vivo substrate specificity of Hu LCAT was assessed by creating animals expressing Hu apo AI + Hu LCAT (HuAI/ LCAT), Hu apo AI + Hu apo AII + Hu LCAT (HuAI/ AII/LCAT), and Hu apo AII + Hu LCAT (HuAII/LCAT). Plasma cholesterol was increased up to 4.2-fold in HuAI/ LCAT transgenic mice and twofold in the HuAI/AII/LCAT transgenic mice, compared with HuAI and HuAI/AII transgenic mice. HDL cholesteryl ester levels were increased more than twofold in both the HuAI/LCAT and HuAI/AII/LCAT mice compared with the HuAI, HuAI/AII, and HuLCAT animals. The HDL particles were predominantly larger in the HuAI/LCAT and the HuAI/AII/LCAT mice compared with those in HuAI, HuAII/LCAT, and HuLCAT animals. The increase in LCAT activity in the HuAI/LCAT and HuAI/AII/LCAT mice was associated with 62 and 27% reductions respectively, in the proportion of Hu apo AI in the pre beta-HDL fraction, when compared with HuAI and HuAI/AII transgenic mice. These data demonstrate that moderate increases in LCAT activity are associated with significant changes in lipoprotein cholesterol levels and that Hu LCAT has a significant preference for HDL containing Hu apo AI.