Avidin biotin technique and lectin assay has been compared in a doubleblind study for detection of the T-antigen in transitional carcinoma of the bladder. Utilizing the tumor registry from the U.S. Naval Hospital in Bethesda, Maryland, patients with transitional cell carcinoma of the bladder were identified. Thirty-three patients were selected with a total of 43 specimens. Tissues from 26 patients with a total of 43 specimens that showed no lesions and had no known history of genitourinary diseases were used as controls. These tissues were studied for the presence of T-antigen utilizing avidin biotin or lectin techniques. T-antigen was detected in 60% of the specimens with lectin technique, while 95% was detected with avidin biotin methods. The T-antigen was not detected by either method even after the treatment with neuraminidase in six patients. These patients had high grade and high stage tumors. This study demonstrated that normal urothelium has concealed T-antigen that can be exposed by treating the tissue with neuraminidase. However, the T-antigen is unconcealed in low grade tumor but disappears in high grade invasive tumor. Avidin biotin technique appears to be a more sensitive method for detecting the presence or absence of the T-antigen.