To develop a serological diagnosis of invasive candidiasis based on detection of circulating secreted aspartyl proteinase (SAP) antigen of Candida albicans, three different enzyme-linked immunosorbent assays (ELISAs) were compared. The first was a standard ELISA to detect anti-SAP antibodies, and the others were an antigen capture ELISA and an inhibition ELISA to detect circulating SAP antigen with monoclonal antibody (MAb) CAP1, which is highly specific for SAP. These tests were applied to 33 serum samples retrospectively selected from 33 patients with mycologically and/or serologically proven invasive candidiasis caused by C. albicans. Serum samples from 12 patients with aspergillosis and serum samples from 13 healthy individuals were also included. The sensitivities and specificities were 69.7 and 76.0% for the standard ELISA and 93.9 and 92.0% for the antigen capture ELISA, respectively. However, these values reached 93.9 and 96.0%, respectively, for the inhibition ELISA. Serum samples from 31 of 33 patients had detectable SAP antigen, with concentrations ranging from 6.3 to 19.0 ng/ml. These results indicate that the inhibition ELISA with MAb CAP1 is effective in detection of circulating SAP antigen and that this assay may be useful for diagnosis and treatment monitoring of invasive candidiasis.