Human carriers of apolipoprotein (apo) A-IMilano are heterozygous for an Arg173-->Cys substitution in the apoA-I primary sequence; despite severe reductions in HDL cholesterol concentrations, affected individuals do not develop coronary heart disease, suggesting that apoA-IMilano may possess antiatherogenic properties. As the beneficial effects of wild-type apoA-I are linked to its role in HDL cholesterol transport, we examined the capacity of apoA-IMilano to recruit cell cholesterol and activate lecithin:cholesterol acyltransferase (LCAT) (two key events in the antiatherogenic reverse cholesterol transport pathway). ApoA-IMilano and wild-type apoA-I were expressed in Chinese hamster ovary cells, and their ability to recruit membrane phospholipid and cholesterol for the assembly of nascent HDL was compared. Both clonal cell lines exhibited similar levels of apolipoprotein accumulation in serum-free medium (approximately 2 micrograms/mg cell protein per 24 hours), and 15% of each apolipoprotein was associated with membrane lipids to form nascent HDL (d = 1.063 to 1.21 g/mL). SDS-PAGE showed that a majority (66 +/- 12%) of the lipidated apoA-IMilano was in the homodimer form. Compositional analyses revealed that apoA-IMilano nascent HDL had a significantly lower (P < .001) unesterified cholesterol/phospholipid mole ratio (0.47 +/- 0.10) than wild-type apoA-I complexes (1.29 +/- 0.14), indicating that apoA-IMilano had a reduced capacity to recruit cell cholesterol. In addition to the reduced unesterified cholesterol/phospholipid ratio, apoA-IMilano nascent HDL consisted mostly of small 7.4-nm particles compared with wild-type apoA-I, in which 11- and 9-nm particles predominated. Despite these changes in nascent HDL particle size and composition, apoA-IMilano activated LCAT normally. We conclude that, even though apoA-IMilano is a normal activator of LCAT, it is less efficient that wild-type apoA-I in recruiting cell cholesterol, suggesting that the putative antiatherogenic properties attributed to apoA-IMilano may be unrelated to the initial stages of reverse cholesterol transport.