Fluorescence was measured in the hydrolysates of several proteins and peptides of known amino acid composition. The most intense fluorescence (emission maximum at 445 nm) was found in acid, but not enzymic, hydrolysates of tryptophan-rich proteins; nine fluorescent fractions were resolved from acid hydrolysates of these proteins by gel filtration. Fluorescence of substances in tryptophan-containing proteins is largely eliminated by reduction, but little affected by oxidation or ultraviolet irradiation. The substances are thermostable and apparently are not amino acids.Tyrosine, tyrosine-containing peptides, and tryptophan-free proteins yielded fewer fluorescent products than did tryptophan-rich proteins. The number and amount of fluorescent derivatives produced from tyrosine during acid hydrolysis depends on the particular protein; also, intra- or intermolecular interactions involving tyrosine or its degradation products appear to influence the nature of fluorescence in protein hydrolysates.The present findings suggest possible quantitation of the tryptophan content of proteins by fluorometric measurement of tryptophan derivatives produced by acid hydrolysis.