Products of radiolysis of tyrosine and glycyl-L-tyrosine in oxygen-free nitrogen, N2O or air saturated water solution, pH 4.0 or 8.6 were analysed in an amino acid analyser and upon separation on DEAE cellulose or BioGel P-2 column with spectrofluorimetry. Apart from dihydroxyphenylalanine tyrosine solution irradiated in nitrogen or N2O contained dityrosine, while that of irradiated glycyl-L-tyrosine contained glycyl-dityrosine-glycine. Both dimeric products were formed with radiation yields, of about 0.15 at low pH value. In addition an unknown, nonfluorescent but ninhydrin positive product was found in irradiated tyrosine solution. Another, unidentified product of radiolysis was detected in N2O or air saturated solutions of tyrosine. The product had a blue-green fluorescence and a molecular weight of about 500.