tert -Butylhydroperoxide (t BOOH) tolerant Candida albicans mutants developed from clinical isolates were characterized with increased tolerance of the oxidative stress generating agents t BOOH and H2O2, continuous induction of the antioxidative defence system, reduced pseudohypha and hypha-forming capabilities, decreased phospholipase secretion and delayed growth in Sabouraud dextrose agar and broth media. Changes in antimycotic (fluconazole, voriconazole, amphotericin B, 5-fluorocytosine) tolerances as well as in total and cytochrome c-dependent respirations showed versatile patterns, meanwhile the intensified alternative oxidase-dependent respiration of the mutants indicated that this respiratory pathway was an important element of the antioxidative defence in general. Because the phenotypes of increased oxidative stress tolerance and reduced virulence attribute production always emerged concomitantly in t BOOH-tolerant mutants the natural selection of C. albicans strains more tolerant of oxidative stress is unlikely. Not surprisingly, a screening study failed to detect any C. albicans strains with increased oxidative stress tolerance among 46 randomly selected clinical isolates.