An antiserum was prepared to a hypotonic saline extract of bladder tumor tissue which, after absorption with normal tissues, was used to localize tumor-associated antigens in a Sephadex G-150 elution profile of urine from patients with bladder cancer. A second antiserum was prepared to the urine fraction with tumor-associated antigens. After absorption with normal urine and plasma this antiserum was reacted in gel diffusion and complement fixation assays against urine samples from normal individuals, patients with bladder cancer and patients with proved urinary tract infections. In the gel diffusion assays none of the 23 urine specimens from normal individuals, 37 per cent of the 24 urine specimens from patients with urinary tract infections, 61 per cent of 13 urine samples from patients with papillomas and in situ tumors, and 95 per cent of 30 samples from patients with bladder cancer gave positive precipitin reactions. In complement fixation assays urine from 53 patients with papillomas through grade IV cancers showed a mean 50 per cent complement fixation at 30 micrograms. protein, 29 patients with urinary tract infections showed a mean 50 per cent complement fixation at 150 micrograms. protein and all but 6 urine specimens from more than 30 normal individuals showed 50 per cent complement fixation (mean value) at 140 micrograms. protein. Thus, an antiserum has been produced that may be used in assays for the diagnosis of bladder cancer.