Undecylprodigiosin is a tri-pyrolle antibiotic made by Streptomyces coelicolor A3 (2). It is the major component (ca. 60%) of a mixture of four red pigments made by the strain, of which butylcycloheptylprodigiosin (ca.34%) is next most abundant compound. Genetic and feeding studies suggest that the compound is made via two convergent pathways, one responsible for production of bipyrollecarboxaldehyde, and the other the monopyrolle moiety. The latter has a fully saturated alkyl chain attached at position 2 of the pyrolle ring. Defined non-polar mutations will be made in each of three sequenced genes that are likely to play a role in mono-pyrolle production (redXWY), and the resulting mutants characterised: by cross-feeding and co-synthesis with previously isolated S. coelicolor red mutants and mutants of Serratia marcescens deficient in prodigiosin production; by analysis of the metabolites they produce; and by mutasynthesis using monopyrolle analogues.