Consideration of structural relationships between the various microbial phenazines, in association with evidence on the manner of shikimic acid incorporation, leads to phenazine-1,8-dicarboxylic acid (I) as a likely common intermediate. Labelled (I) has been found, however, not to be incorporated into pyocyanin (3) in Pseudomonas aeruginosa, phenazine-I-carboxylic acid (4) in P. aureofaciens, or iodinin (6) in Brevibacterium iodinum, and we have been unable to trap (I) during a feeding experiment with P. aureofaciens. Dimethyl phenazine-1,8-dicarboxylate (2), labelled with 14C in the carboxy-groups, showed a total incorporation in excess of 0.2% into (4) and (5) in P. aureofaciens. The dihydro-derivative (7) of (I) was also not incorporated to a significant extent (0.2%) into (4) and (5) in P. aureofaciens. However, (I) showed high incorporations (6.7 and 9.0%) into lomofungin (8) in Streptomyces lomodensis (assayed on the penta-acetate by recrystallization to constant activity), indicating phenazine-1,8-dicarboxylic acid (I) is clearly a lomofungin (8) precursor. We are currently investigating whether (I) is a precursor only for phenazines with two aryl-C1 substituents [as (8)] or whether it is transported across the cell walls of Streptomyces species but not those of Pseudomonas and Brevibacterium species.