The effect of oscillating dissolved oxygen tension (DOT) on the metabolism of Kluyveromyces marxianus was investigated, particularly on the productivity of the enzyme P-galactosidase or lactase (EC 3.2.1.23). Sinusoidal DOT oscillations were established by manipulating the inflowing gases (nitrogen and oxygen), at wave periods of 300, 600 and 1200 s and DOT ranging from 0 to 20%. Cultures carried out at different constant DOT ranging from 0 to 60% of air saturation were used for comparison. Oscillations at the shortest period tested (i.e., 300 s) resulted in higher specific and volumetric enzyme activity (2800 U/g and 31,700 U/l, respectively) and ethanol concentration (12.3 g/l) compared with cultures carried out at longer wave periods (600 and 1200 s) and those cultures carried out at constant DOT (0, 1, 5 and 20%). Final biomass concentration was not affected by oscillating DOT (12.2 +/- 0.7 g/l) and specific growth rate was slightly affected. These findings suggest that some non-ideal DOT control conditions can be advantageous for the case of lactase production, leading to increases in the enzyme volumetric or specific activity, without affecting maximum biomass concentration. (C) 2004 Elsevier Ltd. All rights reserved.