Urine specimens from normal individuals, and from patients with bladder cancer, bladder papillomas and urinary tract infections were assayed for the presence of bladder tumor-related antigens. Ten-fold concentrated urine specimens were reacted in Ouchterlony gel diffusion against various anti-human monospecific antisera. With these antisera urine specimens from normal individuals were distinguished from those from patients with bladder carcinoma as well as bladder papilloma. However, the urine samples from individuals with urinary tract infections showed significant reactivity with many of the monospecific antisera as did specimens from patients with bladder cancer and bladder papilloma. Thus, investigations involved in the assay of bladder cancer biological markers should take the proteinuria associated with urinary tract infection into consideration. The potential importance of detecting tumor-specific components for early diagnosis and treatment of bladder neoplasms is stressed.