To obtain detailed information on the role played by apolipoprotein B (apo B) in determining the structural and functional properties of human plasma low density lipoprotein, we have initiated immunochemical studies of the polypeptide. We report here the establishment of six hybridoma lines that secrete monoclonal antibodies to low density lipoprotein. In addition to recognizing antigenic determinants on low density lipoprotein, all six monoclonal antibodies react with epitope(s) on very low density, but not high density, lipoproteins. The immunoreactivity of these antibodies with low density lipoprotein and with detergent-delipidated apo B was compared in an enzyme-linked immunosorbent assay. Although all six of the antibodies reacted with the apoprotein when it was prepared in a nonionic detergent known to maintain the secondary structure of the protein, three of the six antibodies showed partial or total loss of activity with NaDodSO4- delipidated apo B. The specificity of these antibodies was tested by the ability of affinity-purified biotinylated antibodies to compete with unlabeled antibodies for antigenic sites on low density lipoprotein in a competition enzyme-linked immunosorbent assay developed with avidin-peroxidase. This competition assay allowed us to divide the antibodies into a minimum of two groups (I and II) based on the antigenic determinants on apo B that they recognized. The epitope on apo B recognized by group II antibodies was perturbed in NaDodSO4, whereas the determinant(s) on the protein reactive with group I antibodies was unaffected.