The amount of red pteridine eye pigments(drosopterins) in Drosophila is known to be reduced in the purple mutant and restored to normal levels by a suppressor mutation su(s)2 in suppressed purple mutant. Currently three different types of mechanisms are proposed to explain the su(s)2 suppression; these are tRNA-mediation, transposable element-mediation, and a posttranslational mechanism. To analyzed the suppression mechanism the purple gene product known as 6-pyruvoyl tetrahydropterin synthase (PTP synthase) was purified individually from wild type, purple, and suppressed purple mutant strains and the biochemical properties of each enzyme were investigated. The purple mutant enzyme showed different km value for the substrate dihydroneopterin triphosphate from those of the wild type and suppressed purple enzyme. These results are inconsistent with nonsense suppression and probably with insertional mutagenesis that causes reduced transcription. A mechanism that involves a missense mutation and a posttranslational effect on the mutant enzyme in su(s)± condition is consistent with the data.