Intracellular catalase (EC 1.11.1.6) activity of permeabilized aerobically grown cells of Schizosaccharomyces pombe was insensitive to cell aeration and inhibition of protein synthesis, and was only mildly enhanced by the presence of glucose and ethanol via de novo protein synthesis. By contrast, the intracellular catalase activity of Saccharomyces cerevisiae, which, in freshly harvested cells, was two to three times lower than that in Sch. pombe, increased on aeration without substrates or with ethanol and was inhibited on aeration with glucose following cell permeabilization. The enhanced intracellular activity was due to de novo protein synthesis while the inhibitory effect of glucose, absent in Sch. pombe, was caused by one of the major glucose metabolites, succinate. The intact-cell catalase activity of both yeasts increased greatly during aeration. In Sacch. cerevisiae, this increase was again prevented by glucose. In parallel, export of catalase to the cell surface increased in both yeasts. This was especially conspicuous in Sch. pombe aerated in the presence of ethanol, and may represent a protective mechanism against the damaging effects of ethanol. The cell-surface-bound catalase activity was confirmed in isolated plasma membranes of both yeasts. The fluidity of the plasma membrane increased during aeration. This effect was further stimulated by the presence of glucose and to a lesser extent by ethanol. Both yeasts exhibited increased extracellular catalase activity during aeration which could not be caused entirely by cell lysis. In Sch. pombe this activity was strongly enhanced by the presence of ethanol.