Five lines of transgenic mice, which had integrated the human apolipoprotein (apo) A-I gene and various amounts of flanking sequences, were established. Normally, apoA-I is expressed mainly in liver and intestine, but all of the transgenic lines only expressed apoA-I mKNA in liver, strongly suggesting that 256 base pairs of 5′-flanking sequence was sufficient for liver apoA-I gene expression but that 5.5 kilobase pairs was not sufficient for intestinal expression. Mean plasma levels of human apoA-I varied in different lines from approximately 0.1 to 200% of normal mouse levels. This was not dependent on the amount of flanking sequence. Lipoprotein levels were studied in detail in one of the lines with a significantly increased apoA-I pool size. In one study, the total plasma apoA-I level (mouse plus human) was 381 ± 43 mg/dl in six animals from this line, compared to 153 ± 17 mg/dl in matched controls. Total and high density lipoprotein cholesterol (HDL-C) levels were increased 60% in transgenic animals, compared to controls (total cholesterol: 125 ± 12 versus 78 ± 13 mg/dl, p = 0.0001; HDL-C 90 ± 7 versus 55 ± 11 mg/dl, p = 0.0001). The molar ratio of HDL-C/apoA-I was significantly lower in transgenic animals, 17 ± 1 versus 25 ± 2 (p = 0.0001), suggesting the increase was in smaller HDL particles. This was confirmed by native gradient gel electrophoresis. This was not due to aberrant metabolism of human apoA-I in the mouse, since human apoA-I was distributed throughout the HDL particle size range and was catabolized at the same rate as mouse apoA-I. In another study of 23 transgenic mice, HDL-C and human apoA-I levels were highly correlated (r = 0.87, p < 0.001). The slope of the correlation line also indicated the additional HDL particles were in the smaller size range. We conclude that human apoA-I can be incorporated into mouse HDL, and excessive amounts increase HDL-C levels primarily by increasing smaller HDL particles, comparable to human HDL₃ (HDL-C/apoA-I molar ratio =18).