Following the pioneer work of Schoenheimer, relatively simple organic compounds have, in many instances, been identified as precursors of much larger and more complicated molecules of biological importance. Efforts to bridge the gap between these small organic building stones and the complex final product have, however, met with less success. By investigating the biosynthesis of prodigiosin, the only known naturally occurring tripyrrylmethene, we hoped not only to elucidate this process for its own sake, but also to study it as a possible intermediate or side reaction in the biosynthesis of porphyrins (Rimington, 1949). A preliminary communication on this subject has already appeared (Hubbard & Rimington, 1949). In an effort to explain the occurrence in nature of porphyrins belonging to both the I and III series of isomers, Turner (1940) postulated a tripyrrylmethene as a possible intermediate in porphyrin biosynthesis. His suggestion derived support from the earlier work of Corwin & Andrews (1936), who identified a tripyrrylmethane intermediate in the pyrrol aldehyde synthesis of dipyrrylmethenes and offered an explanation, thereby, of the unexpected occurrence of more than one dipyrrylmethene in the products of the reaction. Since a tripyrrylmethene structure (I) was assigned to prodigiosin by Wrede & Rothhaas (1934), it seemed appropriate to study its biosynthesis and to compare the results with those obtained for haemin biosynthesis by other workers (for summary see Shemin, 1948; Muir & Neuberger, 1948).