Cerulenin, a specific inhibitor of beta-ketoacyl-(acyl carrier protein) synthetase [EC 2.3.1.41] and 3-hydroxy-3-methylglutaryl-CoA synthetase [EC 4.1.3.5], was studied to determine whether it inhibits 6-methylsalicylic acid synthesis, in which so-called "polyketide" formation, a condensation step similar to that in fatty acid synthesis, is involved. In fact, 100 mug/ml (4.5 X 10(-4) M) of cerulenin inhibited 60% of 6-methylsalicylic acid synthetase activity and 68% of fatty acid synthetase activity of Penicillium urticae.