Lipoprotein(a) (Lp(a)) is a lipoprotein formed by the disulfide linkage of apolipoprotein(a) (apo(a)) to the apoB100 of a low density lipoprotein particle. Earlier site-directed mutagenesis studies of apo(a) demonstrated that apo(a) cysteine 4057 is required for the disulfide linkage; however, the cysteine residue within apoB100 that is involved in the disulfide bond has not been identified. We previously demonstrated that the apoB100 produced by human apoB transgenic mice binds to apo(a) and forms Lp(a) (Linton, M.F., Farese, R. V., Jr., Chiesa, G., Grass, D. S., Chin, P., Hammer, R. E., Hobbs, H.H., and Young, S.G. (1993) J. Clin. Invest. 92, 3029-3037). To further explore the structural features of human apoB that are required for the formation of Lp(a), we used a transposon-interrupted human apoB gene clone to develop transgenic mice that express high levels of a truncated form of human apoB, apoB90, which contains the amino-terminal 4084 amino acids of apoB. In vitro incubation of apo(a) with the plasma of human apoB90 transgenic mice did not yield Lp(a), as judged by Western blots of SDS-polyacrylamide gels or by a monoclonal antibody-based radioimmunoassay. In contrast, incubation of apo(a) with the plasma of a mouse that expressed an equivalent amount of the full-length apoB100 did yield Lp(a). In addition to these in vitro incubation studies, no Lp(a) could be detected in the plasma of a "double transgenic" mouse expressing both human apoB90 and apo(a). These data indicate that the carboxyl-terminal 10% of apoB100 contains amino acid sequences that are essential for the formation of Lp(a).