Streptomyces griseus did not produce the polyene macrolide antibiotic candicidin during the initial growth phase characterized by rapid RNA synthesis. the absence of candicidin production when RNA or protein synthesis was inhibited by rifampicin or chloramphenicol suggests a transcriptionally controlled late formation of the candicidin synthases. Phosphate levels in the medium control the rate of DNA, RNA and protein synthesis. Depletion of phosphate appears to trigger the onset of candicidin synthesis after a drastic reduction of the rate of RNA synthesis. Changes in the ATP pool during the fermentation suggest that ATP may be the intracellular effector controlling the onset of antibiotic synthesis.