A recombinant strain of Aspergillus niger (B1-D), engineered to produce the marker protein hen egg white lysozyme (HEWL). was investigated with regard to its susceptibility to oxidative stress. The culture response to oxidative stress. produced either by menadione (MD) or by H2O2, was characterised in terms of the intracellular activities of two key defensive enzymes, catalase (CAT) and superoxide dismutase (SOD), in the presence or absence of a range of vitamin supplements. Addition of either ascorbate (Vit C) or alpha-tocopherol (Vit E) resulted in different responses of A. niger cultures to exogenous H2O2 (1 mM). In the presence of Vit C the activity of SOD was increased by 50% compared to control Cultures, whereas incorporation of Vit E appeared to have a "protective" effect against reactive oxygen species (ROS). Continuous addition of menadione (1.2 mmole l(-1) h(-1)) to submerged cultures of A. niger in the bioreactor resulted in an increase of CAT and SOD activities during the exponential phase of growth, whereas enzyme levels remained almost constant during that phase in control cultures. Comparison of these results with the response to oxidative stress caused by addition of exogenous H2O2 and elevated dissolved oxygen tension (DOT) [1] shows that these three stressors are quite distinct.