The fungal strain Humicola lutea 103 produces a naturally glycosylated Cu/Zn SOD. To improve its yield, the effect of an increased concentration of dissolved oxygen (DO) on growth and enzyme biosynthesis by the producer, cultivated in a 3 l bioreactor, was examined. Exposure to a 20% DO level caused a 1.7-fold increase of SOD activity compared to the DO-uncontrolled culture. Maximum enzyme productivity of SOD was approximately 300 x 10(3) U (kg wet biomass)(-1). The novel enzyme was purified to electrophoretic homogeneity. The presence of Cu and Zn were confirmed by atomic absorption spectrometry. The molecular mass of H. lutea Cu/Zn SOD was calculated to be 31870 Da for the whole molecule and 15936 Da for the structural subunits. The N-terminal sequence revealed a high degree of structural homology with Cu/Zn SOD from other prokaryotic and eukaryotic sources. H. lutea Cu/Zn SOD was used in an in vivo model for the demonstration of its protective effect against myeloid Graffi tumour in hamsters. Comparative studies revealed that the enzyme (i) elongated the latent time for tumour appearance, (ii) inhibited tumour growth in the early stage of tumour progression (73-75% at day 10) and (iii) increased the mean survival time of Graffi-tumour-bearing hamsters. Moreover, the fungal Cu/Zn SOD exhibited a strong protective effect on experimental influenza virus infection in mice. The survival rate increased markedly, the time of survival rose by 5.2 d and the protective index reached 86%. The H. lutea SOD protected mice from mortality more efficiently compared to the selective antiviral drug ribavirin and to commercial bovine SOD. In conclusion, our results suggest that appropriate use of the novel fungal SOD, applied as such or in combination with selective inhibitors, could outline a promising strategy for the treatment of myeloid Graffi tumour and influenza virus infection.