AbstractProduct yield in fermentation processes is often limited by product inhibition or degradation. This may be avoided by extracting the product from the broth during the fermentation. Solvents to be used in these extractive fermentations must be selective for the product and nontoxic for the microorganisms. The procedure of solvent selection is exemplified for the case of formation of cycloheximide by Streptomyces griseus. It is shown that substituted polyglycols, in particular alkylphenol polyethoxylates, are well suited to the requirements of extractive fermentations of antibiotics. In order to prevent foaming the polyglycolether chain of the molecule should contain a few propoxy groups.