The status of Thomsen-Friedenreich antigen (T antigen), in histological sections of human hyperplasia (30 cases) and adenocarcinoma (45 cases) of the prostate, was investigated by the immunohistochemical method. All benign prostatic hyperplasias were negative for T antigen but positive for cryptic T antigen after neuraminidase digestion. Of the 45 cases of adenocarcinoma of the prostate, 27 (60%) were positive for T antigen, 13 (29%) were negative for T antigen but positive for cryptic T antigen, and 5 (11%) were negative T and cryptic T antigens. The status of T antigen was found to be correlated with the histological grade and bone metastasis. However, there was statistically no definitive correlation between the status of T antigen and survival of patients with adenocarcinoma of the prostate.