Silver-staining of immunoprecipitates extends the sensitivity of the radial immunodiffusion assay by tenfold. This modification permits the quantification of apolipoproteins A-I, A-II, C, and E at levels of 0.2-1.0 mg/dl in plasma samples at a sensitivity threshold of 10 ng. The silver-enhanced radial immunodiffusion method is readily adapted from the standard method, simple and inexpensive to perform, and does not require costly instrumentation. These advantages make the modified RID assay an attractive alternative to other forms of immunoassay.