A54145 is a complex of new lipopeptide antibiotics produced by Streptomyces fradiae. Eight factors, containing four similar peptide nuclei in combination with three different fatty acid acyl side chains, have been isolated from the natural fermentation and characterized. The nuclei differ only in valine/isoleucine and glutamate/3-CH3-glutamate substitutions at one or both of two locations on the peptide ring. Prior deacylation of all four nuclei with Actinoplanes utahensis had permitted chemical reacylation of each nucleus with new fatty acid acyl chains for structure-activity relationship studies. In an effort to induce the native biosynthesis of preferred factors or analogs by S. fradiae, the effect of fatty acid precursors on the fermentation was examined. Many fatty acids were extremely toxic to S. fradiae, which limited experiments to slow, continuous feeding of the lipids in stirred bioreactors that were equipped for on-line respiration analysis by mass spectrometry. These studies determined that precursing with aliphatic fatty acids of various chain lengths did enhance the biosynthesis of factors containing specific fatty acid acyl side chains. Caprate, for example, increased the n-decanoyl-containing factors from the natural level of approximately 14% to approximately 80%. The percentage of factors containing branched-chain fatty acid acyl substituents was also increased, in shaken-flask studies, by enriching the medium with valine or isoleucine. These amino acids additionally enhanced the percentage of nuclei containing either valine or isoleucine.