Tryptophan in a concentration of 0.4 microgram/ml increased the production of indolmycin by 37%. The lipophilic character of indolmycin was reduced via directed biosynthesis by substituting the aromatic ring system with a methoxy or hydroxy group in the 5-position of the antibiotic. This substitution was achieved by the addition of the corresponding tryptophan and indole precursors to a growing culture of Streptomyces griseus ATCC 12648. The more hydrophilic indolmycin derivatives displayed a moderate increase in antimicrobial activity as compared to indolmycin, but did not markedly change the Gram-positive/Gram-negative ratio of activity. Thin-layer chromatography and mass spectrometry showed that additives substituted in the 6-position were not incorporated into the molecule. Antibiotic titer was reduced by addition of the modified precursors, especially in the case of the precursors substituted in the 6-position.