Nine lots of raw material salicylazosulfapyridine from six suppliers were analyzed by an early spectrophotometric method. The lower limit of purity set by this method is 88%. Using a highly purified reference standard, results of the spectrophotometric method were compared to a salicylazosulfapyridine-specific, quantitative TLC method. Results confirmed the nonspecificity of the spectrophotometric method. In an extreme case, material with a 94% spectrophotometric purity value was shown to be only 85% by quantitative TLC. As many as eight extraneous spots in addition to salicylazosulfapyridine were found in some lots of raw material when developed chromatographs were sprayed with an azo-indicating stain. The minimum visible detection level of the impurities was between 0.01 and 0.1% of the total salicylazosulfapyridine spotted. Isolated impurities showed spectrophotometric absorbance at the wavelength of the spectrophotometric method, resulting in erroneous purity values. Drug recovery by the quantitative TLC method was 98% with an assay standard deviation of less than ±0.5%. These results led to commercial upgrading of synthesis and purification, so that raw material salicylazosulfapyridine at the 96% level by quantitative TLC was subsequently available for drug production.