A series of sequential mutants from a "wild strain" of tobacco mosaic virus which earlier had shown small serological differences was studied spectrophotometrically for differences in tyrosine and tryptophan content; tryptophan was also determined colorimetrically. No difference in these amino acids as between the mutants was detected. Another group of tobacco mosaic virus strains derived from another wild strain contained more tyrosine than the 1st group, but within the 2d group there was again no difference. The tryptophan content was the same in all strains investigated.