Apolipoprotein B (ApoB) in general is known as an insoluble protein in aqueous buffers without the initial aid of denaturing agents. However, following the total delipidization of rat plasma low-density lipoproteins, a considerable portion (28.2 +/- 3.0%) of rat ApoB was found directly soluble in an aqueous buffer, N-ethylmorpholine acetate (pH 7.3) as demonstrated by SDS-polyacrylamide gel electrophoresis, immunoblotting and electron microscopic analysis. On the other hand, this was not observed for human ApoB. This solubility difference may suggest some structural differences that exist between rat ApoB and human ApoB.