In the hope of obtaining useful probes to study the structure of human apolipoprotein B (apo B), we characterized monoclonal antibodies against low density lipoprotein (LDL). We examined the distribution of their corresponding antigenic determinants on tryptic fragments of LDL separated by monodimensional (SDS) or two-dimensional electrophoresis. Each antibody reacted with several different fragments even when the proteolysis was apparently complete. A peptide of 125,000 daltons was the smallest fragment recognized by all the antibodies. The antibody, 2D8, which cross-reacts with apo B-48 and 3A8 which blocks the LDL pathway both reacted with the same 43,000 dalton fragment. Two other antibodies, 3F5 and 4G3, previously shown to be close together in LDL, also appeared close together in the primary structure of apo B. A determinant present on apo B-26 (1D1) was dissociated from all others examined on fragments of less than 125,000. Similarities in the patterns of reactivities with LDL-tryptic fragments between certain monoclonal antibodies and the lectins Concanavalin A and Limax flavus agglutinin indicated the proximity of the corresponding antigenic determinants to carbohydrate moieties. Competition studies suggested that the two major carbohydrate chains of LDL do not participate in the determinants themselves.