AbstractObservations on quantitative relationships between oxygen diffusion rate and product formation were made with penicillin, bacitracin, and gluconic acid. For penicillin and gluconic acid formation, no toxic oxygen concentration could be achieved in the broth, whereas concentration above approximately 7 mg. O2/l. suppressed the bacitracin yield to an ever increasing extent. The intensity of mixing was measured by means of distributing a dye. With penicillin formation as an example, it was shown that the antibiotic yield is not only dependent on maintenance of a certain oxygen diffusion rate, but also on ensuring a necessary minimal mixing.