<jats:p> When <jats:sc>dl</jats:sc> -isoleucine was added to an ammonium nitrate-maltose medium during cultivation of <jats:italic>Streptomyces</jats:italic> sp. 732, quinomycin B formation was selectively enhanced (from 3 to 70% of the quinomycin mixture) and quinomycin C synthesis was inhibited completely. In addition, two new quinomycins, designated quinomycins D and E, were produced in the presence of isoleucine. These compounds were found to contain <jats:italic>N</jats:italic> -methylalloisoleucine. In experiments with isoleucine enantiomorphs, it was determined that the order of effectiveness for quinomycin B synthesis was <jats:sc>dl</jats:sc> -isoleucine > <jats:sc>d</jats:sc> -isoleucine > <jats:sc>l</jats:sc> -isoleucine. The extent to which quinomycin B synthesis is enhanced depends upon the concentration and the time of addition of isoleucine to the medium. The effect of <jats:sc>dl</jats:sc> -isoleucine was reduced to some extent by the addition of <jats:sc>l</jats:sc> -valine. It is conceivable that amino acids which are precursors of the <jats:italic>N</jats:italic> -methylamino acids in quinomycin can regulate quinomycin formation.