Addition of cyclic(c)AMP to Streptomyces griseus fermentations inhibited candicidin formation. In a phosphate-free resting cell system cAMP inhibited net candicidin formation and incorporation of labeled propionate and p-aminobenzoic acid into the antibiotic but did not inhibit protein synthesis. All nucleotides tested, regardless of the position of the phosphate ester, were effective inhibitors; nucleosides and free bases were not. Inhibition occurred whether the nucleotide was added early or late. Inhibition of antibiotic formation by exogenous nucleotides, including cyclic nucleotides, is similar to the effect produced by Pi.