Apolipoprotein B obtained from each of the major density classes of human plasma lipoproteins was investigated immunochemically. Eleven different mouse hybridoma cell lines were generated and selected on the basis of their ability to secrete antibodies that were bound by either very low density or intermediate density lipoproteins. Two of these antibodies appeared to be specific for complex epitopes on apoprotein B, since they were bound by all apoprotein B-containing lipoproteins, but not by apoprotein B after it had been delipidated and electrophoresed on sodium dodecyl sulfate-polyacrylamide gels. The remaining nine hybridoma antibodies were capable of binding to apoprotein B; and appeared to be specific for relatively simple epitopes that were expressed by denatured apoprotein B. The ability of these nine hybridoma antibodies to bind to the denatured apoprotein B species of plasma chylomicrons, very low density lipoproteins, and low density lipoproteins was compared. The apoprotein B species were separated on the basis of their relative mobility in low percentage sodium dodecyl sulfate-polyacrylamide gels and transferred to nitrocellulose paper for reaction with antibody. These binding studies suggest that plasma apoprotein B represents a complex family of immunochemically heterogeneous but related apoprotein species. A schematic summary of the apoprotein B species distribution of epitopes identified by these hybridoma antibodies is presented.