Enzyme immunoassays (EIAs) using rabbit anti-gamma-seminoprotein (gamma-Sm) antibody and rabbit anti-prostate-specific antigen antibody (PA) for measurements of gamma-Sm and PA were developed in order to determine the correlation between serum gamma-Sm levels and serum PA levels in patients with prostate cancer. Each EIA for measurement of gamma-Sm or PA was revealed to be sensitive and reproducible by fundamental analysis of the quality of EIAs. A significant correlation was established between serum gamma-Sm levels and serum PA levels which were measured by these assays and expressed as protein concentration of seminal plasma (r = 0.99). This result coincided with our previous observation that gamma-Sm and PA are immunologically identical materials. We conclude, therefore, that gamma-Sm and PA should be regarded as tumor markers identical to each other for serodiagnosis of prostate cancer.