Culture (NAD(P)H) fluorescence dynamics have been used to provide information on culture behaviour when Xanthomonas campestris was grown in a bioreactor. Culture fluorescence decreased by 1150 units in response to an increase in extracellular pH from 3.1 to 7.6. A mathematical model incorporating the effect of pH on the bulk NADH depletion reaction simulated the experimental data. The rates of bulk NADH formation and depletion reactions were 1 s(-1) and 719 (M h (1))(-1) s(-1), respectively. Subsequent to the initial NADH decrease, the culture fluorescence increased to within 200 units of its original value, with a concomitant decrease in oxygen uptake rate (OUR) from 7.3 to 3 mM h(-1). A mathematical model incorporating the hypothesis that the culture manipulated its OUR to increase its NADH level, simulated the experimental data. In addition, it was inferred from culture fluorescence that the intracellular oxygen availability becomes insufficient at or below 10% extracellular dissolved oxygen value. Studies on H2O2 addition to X. campestris to optimize the liquid-phase oxygen supply, showed no change in metabolic state, as indicated by NADH fluorescence, until 1.4 mmol H2O2 (g cell) (1) and a significant decrease above that. Investigations on the reasons for decreases in NADH fluorescence suggested a DNA-damaging Fenton reaction as the probable reason for the observed NADH decrease on addition of H2O2.