We have characterized the lipoproteins isolated from the plasma of a human subject who was heterozygous for a mutation yielding a truncated apolipoprotein B (apo-B32). The apo-B32 lipoproteins were isolated from the plasma high density lipoprotein (HDL) fraction by heparin-Sepharose chromatography. Although the chemical composition of the apo-B32-containing lipoproteins was similar to that of normal HDL, the mean diameter of the apo-B32 lipoproteins was larger than typical apo-AI-containing HDL particles. On agarose gels, the apo-B32 lipoproteins had pre-beta mobility similar to that of normal very low density lipoproteins. Analysis of the purified apo-B32 lipoproteins by SDS-polyacrylamide gel electrophoresis revealed the presence of both apo-AI and apo-E. In order to further analyze the properties of apo-B32, we developed an apo-B32 expression vector and generated stable rat hepatoma cell lines expressing apo-B32. In these cell lines, the apo-B32 protein was secreted in a d > 1.21 g/ml lipoprotein. Oleic acid supplementation of the cell-culture media had no measurable affect on the density distribution of the apo-B32 lipoproteins that were secreted by the cells.