Cultures of Streptomyces griseus grown under phosphate-limiting conditions produced a complex of green products. Three of these were separated from the mixture and characterized. One was identified as viridomycin A, the ferrous chelate of 4-hydroxy-3-nitrosobenzaldehyde; the second (actinoviridin A) was the corresponding carboxylic acid chelate and the third (viridomycin E) was a hybrid chelate containing both the aldehyde and acid ligands. Only two out of nine strains of S. griseus examined produced viridomycins and the ligands were biosynthesized only in media from which phosphate had been exhausted. Optimization of the production medium showed that fructose and alanine were the most favorable carbon and nitrogen sources and that relatively high concentrations of ferrous ions were necessary. The results suggest that viridomycins are not produced by S. griseus as iron scavengers in response to iron deficiency but as secondary metabolites that are stabilized adventitiously in the broth by metal ion chelation.