Addition of glucose to glucose-derepressed yeast cells causes disappearance of 60 % of the activity of fructose-1,6-bisphosphatase within 3 to 5 min. Reversibility of this "catabolite inactivation" reaction in a glucose-free medium is independent on de novo protein synthesis. The pH-optima of fructose-1,6-bisphosphatase activity in gel-filtrated crude extracts were shown to be 8.25 for the enzyme from derepressed cells and 8.8 for the enzyme from cells treated with glucose for 4 min. In studies with |3H| - leucine labelled glucose-derepressed cells the protein cross reacting with antibodies against fructose-1,6-bisphosphatase did not disappear within the first 10 min after addition of glucose. These findings suggest that the glucose induced rapid inactivation of the enzyme is the result of a covalent modification which decreases the fructose-1,6-bisphosphatase activity and changes the pH-activity profile of the enzyme, but does not change its immunological reactivity to antibodies. It is concluded that the covalent modification renders the enzyme susceptible to proteinases and thereby initiates its selective proteolysis.