Cultures ofStreptomyces venezuelaepresented with a mixture of ammonium and an amino acid as nitrogen sources used both compounds together. Absence of ammonium repression of alternative nitrogen assimilation pathways was also observed when ammonium was added to cultures already growing on proline. The presence of ammonium in the mediumab initiodepressed the yield of chloramphenicol. However, its addition to a culture growing on proline caused only a temporary inhibition of antibiotic synthesis, even when sufficient ammonium was added to create an excess. Continuous cultures supplied with ammonium as the growth-limiting nutrient showed no significant change in specific antibiotic production at different specific growth rates. The overall results indicate that inS.venezuelaeneither nitrogen utilization pathways nor chloramphenicol biosynthesis is controlled by nitrogen repression.