We describe two new monoclonal antibody-based, solid-phase immunoenzymometric assays for the quantification of apolipoprotein (apo) B in plasma: a competitive assay and a direct assay. For both, we utilize 96-well microtiter plates and native low-density lipoprotein (LDL) for preparing the standard curve. A single monoclonal antibody, MB24, is used in the competitive assay. The direct assay involves use of two monoclonal antibodies, MB24 and MB47. These two antibodies bind to distinct, unrelated apo B epitopes expressed by all LDL particles, and both antibodies detect apo B in very low-density and intermediate-density lipoproteins as well as LDL. With the two-step competitive assay, which involves use of LDL-coated microtiter plates, the intra- and interassay reproducibility in plasma apo B measurements averaged 6% and 12%, respectively. In the one-step direct assay, which takes less than 2 h for completion, plasma samples and peroxidase-labeled MB24 are incubated simultaneously on MB47-coated microtiter wells. The amount of labeled MB24 bound to lipoproteins trapped by MB47 is proportional to apo B concentration. With the direct assay, intra- and interassay CVs averaged 7% and 12%, respectively. These assays are simple, reproducible, involve convenient incubation intervals, and do not require radioisotopes; thus they can be widely applied in clinical laboratories.