Rat hepatoma McA-RH7777 cells synthesize and secrete two populations of apoB-containing lipoproteins: a larger, VLDL-sized population floating in the Sf 40-150 range and a smaller, LDL and HDL-sized population. Three permanently transfected cell lines of McA-RH7777 cells secreted (in addition to the endogenous lipoproteins) lipoproteins containing 1) a carboxyl-terminally truncated human apoB-53 (2377 amino acids in length); 2) a carboxyl-terminally truncated human apoB-31 (1420 amino acids in length); or 3) an internally deleted human apoB protein, apoB-18/95, containing a total of 1490 amino acid residues, equivalent in length to an apoB33. The apoB-18/95 protein contained amino acid residues 1-782 joined to 708 residues near the C-terminus of apoB (residues 36364343). All three of the apoB peptides, apoB53, apoB-31, and apoB-18/95, were present on smaller LDL-HDL-class lipoproteins, with buoyant densities in the HDL density range. The sizes of the HDL class lipoproteins agreed with prior observations that lipoprotein core circumference is directly proportional to apoB size. As HDL containing apoB-18/95 conformed to this rule, contiguous apoB amino acid sequence is not required for the rule to be obeyed. In addition, apoB-18/95, but not apoB-31, was also present on the VLDL-sized lipoproteins even in the absence of serum or oleate supplementation. As the latter two constructs encode equally sized apoB peptides, their particular amino acid sequences rather than just overall length must determine whether they can assemble into a VLDL particle.