We describe a double-antibody radioimmunoassay for apolipoprotein B with use of monoclonal antibodies. Values for 32 patients with various lipid disorders, obtained with use of one of the monoclonal antibodies (464B1B6), were similar to those obtained with use of a polyclonal antiserum. When we used another monoclonal antibody (465C3D1) the values were significantly lower. Monoclonal antibody 464B1B6 recognized apolipoprotein B determinants in the very-low-density-lipoprotein and low-density-lipoprotein fractions of plasma in a similar fashion as an unfractionated plasma, but monoclonal antibody 465C3D1 gave lower values with the fractionated plasma. For precipitation of the antibody-apolipoprotein complex to be complete, use of polyethylene glycol was necessary with one of the monoclonal antibodies but not with the other. The interassay CV was 12.3% and 11.6% for the assays with the monoclonal antibodies. Analytical recoveries ranged from 85 to 94%. Owing to variations in the B50 from one preparation of standard to another, we found it necessary to normalize the results to those of a serum pool. We conclude that certain monoclonal antibodies can be used effectively in radioimmunoassays for apolipoprotein B in plasma and may help in the standardization of these assays.