A comparison of paraprotein estimation by scanning densitometry with corresponding immunoglobulin concentration measured by rate immunonephelometry, fixed-time immunonephelometry and kinetic immunoturbidimetry using a large number of sera has indicated overestimation of the immunoglobulin by liquid phase immunoassays in the presence of a monoclonal component. The effect was seen with all three major immunoglobulin isotypes but was most marked with IgM paraproteins when measured by kinetic nephelometry. The overestimations could not be explained by presence of rheumatoid factors or immune complexes in the serum samples, or by non-parallelism, over a range of dilutions, of the reaction of sample and calibrant with the antiserum. The source and species of antiserum used was a major contributory factor.