Delipidation by ethanol-diethyl ether at -10 degrees C of human serum high-density lipoprotein (HDL, d 1.063-1.21) or of its subclasses HDL(2) (d 1.063-1.120) and HDL(3) (d 1.120-1.21), yielded proteins-alphaP, alphaP(2), and alphaP(3)-containing 3% phospholipid (largely lecithin) and 3.3% carbohydrate (glucosamine:L-fucose:D-galactose, D-mannose:sialic acid, 1.00:41 : 0.56:0.31). Solubility data and analytical ultracentrifugal analyses indicated that, upon lipid removal, HDL protein aggregates readily; the aggregation is dependent upon pH and ionic strength of the solvent medium. Subunits of 21,000 mol wt were obtained by acetylation or addition of sodium dodecyl sulfate (SDS). HDL and alphaP elicited in the rabbit a similar immunological response. By agar gel immunoelectrophoresis both anti-HDL and anti-alphaP sera detected a major and two minor antigenic determinants in HDL, HDL(3), alphaP, alphaP(2), and alphaP(3). HDL(2), antigenically homogeneous, gave an immunoelectrophoretic pattern of HDL(3) upon mixing with alphaP. alphaP, alphaP(2), and alphaP(3) exhibited a single antigenic determinant after treatment with SDS (0.5 M) or upon acetylation. Native or delipidated forms of HDL, HDL(2), and HDL(3) were separated by vertical starch gel electrophoresis into several components, which showed identical reactions against anti-HDL or anti-alphaP sera. The data suggest that (a) the proteins of HDL, HDL(2), and HDL(3) are made of subunits, probably identical, of an average molecular weight of 21,000; (b) the difference in antigenic behavior between HDL(2) and HDL(3) is due to the presence in the latter of a lipid-poor protein; (c) antigenic polymorphism of alphaP is probably related to the presence in solution of monomeric and polymeric forms having different reactivity against anti-HDL and anti-alphaP sera.