A mutant strain of Streptomyces hygroscopicus was found to produce up to 9.0 units/L of an immunoregulant precursor, immunomycin, with up to 3.5% of a lower homolog impurity under either dual fed-batch or batch conditions. Glycerol and valine were key nutrients influencing productivity and impurity levels. Soybean oil was successfully substituted for glycerol as a carbon source to minimize shot additions to batch culture. The remainder of the production medium was composed largely of defined components with the exception of yeast extract.Valine limitation increased lower homolog formation while decreasing higher homolog formation; excess valine decreased lower homolog formation below 2-3% while increasing higher homolog formation. Higher homolog formation in the presence of valine seemed to be slower than lower homolog formation in the absence of valine. Valine was believed to be the major butyrate precursor; consequently its availability influenced the impurity profile. A preliminary cost analysis suggests that elimination of added valine from the cultivation and replacement of glycerol with soybean oil can result in a 6.6-fold reduction in media costs relative to the original fed-batch process.