Enzymatic dehydrogenation of tryptophan residues in human alpha- and beta-globins has been demonstrated with tryptophan side chain oxidase II, a hemoprotein from Pseudomonas. In 1 M ammonium acetate, pH 4, the enzyme catalyzed the conversion of Trp-14 of alpha-globin to alpha, beta-dehydro-Trp. Under identical conditions, Trp-15 of beta-globin was converted to alpha, beta-dehydro-Trp, whereas Trp-37 was unmodified. No modification was obtained in 0.4% sodium dodecyl sulfate or 8 M urea. The dehydrogenase component of tryptophan side chain oxidase II was also active toward globins in the presence of ferricyanide, whereas tryptophan side chain oxidase I was inert. Upon analyses of the tryptic fragments of the modified beta-globin, two forms of alpha, beta-dehydro-Trp-15 distinguishable by ultraviolet absorption and fluorescence were identified. Each of them rapidly equilibrated with another form on irradiation at 300-380 nm. We tentatively assigned them to two isomers of alpha, beta-dehydro-Trp distinguishable by the configuration around its beta-carbon atom. alpha, beta-Dehydro-Trp in globins and certain polypeptides used herein showed a blue fluorescence at 420 nm. In [alpha, beta-dehydro-Trp 15]beta-globin, the fluorescence was exceptionally intense, the fluorescence yield being comparable to that of NADH. From several lines of evidence, the result was ascribed to the excitation energy transfer from Trp-37 to alpha, beta-dehydro-Trp-15 in the modified beta-globin.