The experiments in this paper have been chiefly concerned with a derivative from pterins, the wing pigments of pierid butterflies. These pigments have become prominent of recent years, partly on account of the prolonged efforts (1925-41) of continental workers to determine their nature. The progress of these constitutional studies has quite recently become dramatic, as is made clear in the historical introduction below. Moreover, one pterin at least has been shown to occur in mammalian tissues, with probable physiological functions there. From one pterin (erythropterin) the yield of the derivative with which the present study has dealt is large. From another (xanthopterin) it arises only in special circumstances which are of considerable interest. The substance gains in interest from its mode of origin. This involves an oxidation by molecular oxygen in solutions of low pH. On the acid side of neutrality its formation proceeds throughout a wide range of pH values, but not at all on the alkaline side. The properties and behaviour of this derivative (which in some respects are exceptional) are described, and among the experimental results obtained are some which offer guidance for future extension. The ground covered has been limited, however, by difficulties at the present time in obtaining an adequate supply of material. No evidence bearing on the actual constitution of the product has yet been obtained, and the paper represents what is essentially a preliminary study. It seems justifiable to claim, however, that until the structure of this derivative is determined and the mechanism of its formation made clear, the chemistry of the pterins will remain incomplete. Another section of this paper deals briefly with the yellow products which occur when aqueous suspensions of uric acid are heated in sealed tubes at high temperature. These show properties strikingly akin to those of the pterins, and in particular yield purple derivatives in precisely those circumstances which lead to the formation from the natural pigments of the product here dealt with which they closely resemble.