The yeast Saccharomyces cereoisiae produces a cytochrome P-450 enzyme which is thought to catalyse the Id Yoshida & Aoyama, 1984). Thus oxygen may have an important role in regulating cytochrome P-450 biosynthesis. S . cereoisiae (NCYC 754) was grown at 30°C in an orbital shaker at a series of different shake speeds. The growth medium consisted of 20% (w/v) glucose, 2% (w/v) mycological peptone, 1% (w/v) yeast extract and 0.5% (w/v) sodium chloride. Yeast was inoculated with a wire loop into IOOml of medium in 250ml flasks. Dry weight estimates were obtained by removing samples from the culture, washing the yeast with distilled water and drying at 70°C to constant weight. Cytochrome P-450 was determined directly on whole cells using its reduced carbon monoxide difference spectrum (Omura & Sato, 1964). Fig I shows the effect of growing S. cereoisiae in batch culture at a series of shake speeds on the accumulation of cytochrome P-450 and the growth of yeast. A clear maximum cytochrome P-450 level is observed at 120rev./ min, with the enzyme level per g of yeast decreasing at both slower and faster shake speeds. The maximum yield of cytochrome P-450 occurs at the same shake speed as the maximum growth rate and also the maximum yield of yeast biomass. This suggests a close link between a high yield of cytochrome P-450 and a high growth rate. These interesting findings with the orbital shaker are related to the known effect of agitation in giving enhanced transfer of solutes between the microbial biomass, the bulk medium and the associated gas phase. Transfer rates generally increase with increasing agitation, and an optimal agitation intensity may be expected to correspond to the optimal level of a particular solute. In parallel work, using stirred-tank fermentation (Salihon et al., 1985), cytochrome P-450 yield from S. cereoisiae NCYC 754 showed a clearly defined optimum for agitation. With growth conditions optimized using air as oxygen source, growth under identical conditions, but using air mixed with either nitrogen or oxygen, gave severely reduced enzyme yields, indicating that the agitation optimum found in this work indeed corresponds to optimal dissolved oxygen availability. Blatiak et al. (1983) have previously reported that small h