Royal Purple, the most famous indigoid dye of antiquity, was derived from hypobranchial gland extracts of various marine gastropod mollusks. The extracts were processed by a lengthy, elaborate procedure, according to the Roman writer of the 1st century A.D., Pliny the Elder; other, more cursory texts are known from the Classical Greek period up until Byzantine times. To obtain fast, intense textile colors, as described in the ancient texts, a soluble precursor or derivative must have been used, which was then converted to the indigoid dye processing, and in the formation of Royal Purple from hypobranchial gland extracts suggests how this might have been accomplished by controlling the oxidative, photochemical, and/or enzymic/hydrolytic pathways to the dye. Because of the imprecision and ambiguity of the ancient texts, however, the dyeing process can only be reconstructed by assuming that certain additives entered into specific reactions (e.g., an antimony (not lead) reducing system in Pliny's account). Inferences are even more difficult to draw about the earliest known Royal Purple dye processing, based on the archaeological evidence from a 13th century B.C. industrial context at the site of Sarepta in Lebanon.