Gamma-seminoprotein (gamma-Sm), a potential new marker for prostate cancer, has been evaluated with a sandwich-type enzyme immunoassay (EIA). This assay system has been confirmed to have a sensitivity and detectable range of 3.0 and 3.0-100 ng/ml, respectively, with a high reproducibility (approximately equal to 6% coefficient of variation between assays). A total of 256 serum samples were drawn from normal Japanese subjects for detection of gamma-Sm. Serum gamma-Sm was undetectable (less than 3.0 ng/ml) in 26 samples from 26 females. In 230 male cases, serum gamma-Sm levels ranged from less than 3.0 to 4.0 ng/ml. These values were not related to age. An upper normal limit of 3.6 ng/ml was calculated for 99 percentile Japanese males (n = 103) over 50 years of age. Serum gamma-Sm was detected in 192 untreated male patients with urological diseases. Gamma Sm levels (mean +/- SD) in each disease were as follows: prostate cancer (n = 64) 11.0 +/- 17.9 ng/ml; benign prostatic hypertrophy (n = 50), 3.02 +/- 0.113; bladder cancer (n = 58), 3.13 +/- 0.514; and renal adenocarcinoma (n = 30), 3.26 +/- 1.01. Serum gamma-Sm levels were statistically higher (p less than 0.05) in the prostate cancer group, however, there was no statistical difference in gamma-Sm levels among clinical stages or histopathologic grades. Furthermore, serum gamma-Sm values showed no correlation (r = 0.3870) with prostatic acid phosphatase (PAP), but were slightly correlated to prostate antigen (PA) levels (r = 0.6980) in patients with prostate cancer. These results suggest that gamma-Sm is a potential tumor marker of prostate cancer and that serially detected serum gamma-Sm levels could be used to monitor the disease.