SUMMARY: Under the conditions prevailing during the early stages of certain higbIy aerated penicillin fermentations, lard oil and oleic acid appear to be oxidized only slowly, or not at all, by suspensions of washed mycelium of Pen4cilliUrn chryso- gmum. Lard oil and oleic acid are both oxidized rapidly, however, under the con- ditions present at 48 hr. and after. The optimum pH for oxidation of lard oil by suspensions of washed mycelium c. 48 hr. old is 8.1 and for oleic acid, c. 64, but oxida- tion of both lard oil and oleic acid was apparent at the pH prevailing during the early stages of the fermentation (545-6-0). Mycelium from shaken flask fermentations (conditions of relatively low aeration) appeared to oxidize lard oil only slowly even during the later stages of the fermenta- tion although oleic acid, which is the principal product of hydrolysis of lard oil, was oxidized rapidly. The results suggest that enzymes involved in the oxidation of lard oil and oleic acid may be present only in relatively low concentration in young mycelium and be synthesized as growth takes place. The ability to oxidize lard oil also appears to be favoured by conditions of high aeration during growth. It has been shown (Rolinson & Lumb, 1953) that in highly aerated spore- inoculated penicillin fermentations, lard oil, added as a vehicle for the anti- foam agent, is utilized as a major source of carbon by Penicillium chrysogenum. Suspensions of washed mycelium from similar highly aerated fermentations were also capable of oxidizing lard oil rapidly when it was added as the sole substrate, but washed mycelium grown under conditions of relatively low aeration (in shaken flasks) failed to show any increase in respiration over the endogenous rate when incubated with lard oil. The present paper is concerned with further investigations upon factors affecting this oxidation of lard oil by P. chrysogenum. METHODS Fermentation methods. Two types of fermentation equipment were used which provided widely different conditions of aeration. Conditions of high aeration were obtained in laboratory fermenters of the type described by Lumb & Fawcett (1951) containing 3 1. medium with stirring at 1000 r.p.m. and an air flow of 6 l./min. Relatively low aeration was obtained by using 75 ml. medium in 250 ml. conical Pyrex or Hysil flasks on a rotary shaker having a speed of 220 r.p.m. and a throw of 1Q in. All fermentations were carried out at 24-26'. The medium used in all the experiments contained: corn steep liquor (solids basis), 1-5 % ; lactose, 3.0 % ; glucose, 0.5 % ; NaNO, , 0-3 % ; =PO,, 0.05 % ; Na$O,, 0.1 % ; pH adjusted to 5.8. Phenylacetic acid was added to the stirred and aerated fermentations in additions of 0.05% every