The human apolipoprotein apoAI, apoCIII, and apoE genes were placed under the control of the mouse metallothionein 1 promoter in a bovine papilloma virus vector that also contained the human metallothionein 1A gene. Following transfection of mouse C127 cells with the expression vector, cell clones resistant to Cd2+ were selected and found to express in high abundance specific apolipoprotein genes. Individual cell clones expressing apoAI, apoCIII, or apoE genes were used further to study the isoprotein composition and the flotation properties of the corresponding nascent apolipoproteins. It was found that the lipoproteins secreted by cell clones expressing the apoAI, apoCIII, and apoE genes consisted of the proapoAI disialylated form of apoCIII (apoCIIIS2) and mainly sialylated forms of apoE. Separation of the secreted apolipoproteins by density gradient ultracentrifugation resulted in limited flotation of nascent apoAI, apoE and apoCIII in the high density lipoprotein (HDL) fraction. Similar analysis in the presence of human serum increased the flotation of apoAI, apoE, and apoCIII to 6.5-, 4.5-, and 5.5-fold, respectively, and resulted in their redistribution to various lipoprotein fractions. HDL increased the flotation of apoAI to 12-fold and very low density lipoprotein (VLDL) increased the flotation of apoCIII and apoE to 6.5- and 5.5-fold, respectively. These findings suggest that in the cell system used, the majority of nascent apoAI, apoCIII and apoE is secreted in the lipid-poor form, which then associates extracellularly with preexisting lipoproteins.