Background: Some clinical and experimental questions require quantitation of soluble serum proteins in amounts lower than those which are detectable by radial immunodiffusion (RID) or nephelometry. Materials and Methods: We describe a competitive enzyme-linked immunoassay (CELIA) for the quantitation of minor amounts of soluble human IgG and albumin. Results: The assay enables the detection of IgG and albumin in the range of 5 to 125 ng/ml. Conclusions: Besides its high sensitivity, the main advantage of this double-step technique is that the sample-specific primary antibody is not needed in an enzyme-conjugated form because of the use of a labelled secondary antibody.