The production of monoclonal antibodies to human lipoproteins is described. One of these antibodies, which was shown to be specific to apolipoprotein B, was used to develop a competitive enzyme-linked immunoassay for apolipoprotein B in serum samples. The antibody selected recognises apolipoprotein B in both low density and very low density lipoprotein particles, but there is no cross-reactivity with high density lipoprotein. There is no requirement for labelling of antigen or antibody used in the assay, and results obtained correlate well (r = 0.88) with measurements on serum samples using a radial immunodiffusion assay for apolipoprotein B.