We analyzed 55 spot urine samples from patients with cardiovascular diseases. Urinary albumin concentrations were measured with size exclusion high performance liquid chromatography (HPLC), turbidimetric immunoassay (TIA) using anti human serum albumin polyclonal antibody, and enzyme-linked immunosorbent assay (ELISA) using anti human serum albumin monoclonal antibody. Fractionated urine samples from the HPLC were also analyzed with the immunoprecipitation reactions using as same monoclonal antibody as ELISA. As a result, the urinary albumin concentration analyzed by the HPLC was systematically higher than that of immunoassays, however, 'albumin peak' from the HPLC contained other urinary proteins. Result of immunoprecipitation reaction showed the presence of monomer albumin that could not react with the monoclonal antibody. These results suggest that not only contamination of other proteins, the albumin fraction from the HPLC included albumin with reduced its reactivity to the specific monoclonal antibody.