The effect of a D-glucose on growth and erythromycin production by Streptomyces erythraeus was investigated. D-Glucose stimulated growth and caused a strong but temporary suppression of antibiotic formation. Maximum specific suppression of erythromycin formation occurred at a carbohydrate concentration of 20 mg ml-1. A non-metabolizable analogue of glucose, 2-deoxy-D-glucose, also suppressed antibiotic formation. Since glucose caused a decrease in erythromycin formation only when added before the stage of antibiotic production, we conclude that this sugar exerted a transient repressive effect on erythromycin biosynthesis.