Mushroom tyrosinase and the recently identified, 4-alkyl-o-benzoquinone: 2-hydroxy-p-quinone methide isomerase were used to investigate the mechanism of biosynthesis of papiliochrome II pigment found in the yellow scales of the papilionid butterflies. Incubation of N-beta-alanyldopamine (NBAD) and L-kynurenine with mushroom tyrosinase resulted in the formation of adducts tentatively characterized as NBAD quinone-L-kynurenine adducts. If quinone isomerase was included in this reaction mixture, the formation of two new products could be witnessed. These two products exhibited the same retention time and the same UV and visible spectral properties as those of papiliochrome II diastereoisomers. Since quinone isomerase catalyzes the conversion of quinones to quinone methides, the above studies indicate that papiliochrome II biosynthesis involves non-enzymatic and hence non-stereoselective condensation of enzymatically generated NBAD quinone methide with L-kynurenine.