Ultraviolet fluorescence was studied in specimens of bladder tumor tissue removed from 29 patients who had received tetracycline or methacycline, and from 8 patients who had received no tetracycline compound. Fluorescence was observed by macroscopic and microscopic examination and by spectrophotometry of tumor extracts. Tetracycline compounds imparted a bright yellow fluorescence to calcium salts encrusted upon the tumor but in only 3 cases was any similar degree of fluorescence observed in tumor tissue itself. Spectrophotometry has shown that the auto-fluorescence of bladder tumor tissue occurs at a similar wavelength to tetracycline fluorescence. Lesser degrees of yellow-green fluorescence of bladder tumors cannot be regarded as specific to tetracycline, therefore. Visual qualitative assessment of ultraviolet fluorescence in bladder tumors after tetracycline administration does not appear to provide a satisfactory basis on which to establish a reliable diagnostic method.