The suppressor gene, su(s)², in Drosophila melanogaster restores the production of brown eye pigment in the vermilion mutant by partially restoring tryptophan oxygenase activity. Our data indicate that the product of the su(s)^+ allele inhibits the activity of the mutant tryptophan oxygenase but not that of the wild type enzyme. An epigenetic reactivation of brown eye pigment formation was shown to be reduced by increasing the number of su(s)^+ copies in vivo. These data support a previously proposed mechanism for suppression by su(s)², which postulates that the enzyme produced by a mutant form of a suppressible gene (vermilion in this case) is inhibited or inactivated by the product of the su(s)^+ allele. When the su(s)² allele replaces su(s)^+ the inhibitory product is either absent or ineffective and the tryptophan oxygenase activity reappears.