Two varieties of enzyme repression have been observed in micro-organisms. In one, addition of the end-product of a biosynthetic pathway to the growth medium specifically prevents synthesis of enzymes concerned with the formation of the product in question. In the other, synthesis of a degradative enzyme is repressed by an end-product (or a closely related metabolite) of the action of the enzyme. Several authors^1,2 have suggested that a repressing metabolite may exert its effect by combining with newly formed (â nascentâ ) enzyme and, thereby, somehow preventing its release from the site of synthesis. Extending this notion, it could be supposed that enzyme formed under non-repressed conditions might still retain â recognitionâ sites accessible to a repressing metabolite. Binding of the metabolite to these sites might then be expected to alter certain properties of the enzyme, such as heat stability. It is commonly observed that the heat stability of an enzyme is altered when it is in combination with a substrate or an inhibitor. As an approach to testing for the possible existence of â repression recognition sitesâ , we have examined the effects of metabolites derived from glucose on the heat stability of Escherichia coli β-galactosidase. The synthesis of this degradative enzyme, as well as several others, in E. coli is repressed when glucose is present during growth and can be similarly affected, though to a lesser extent, by any compound that can be used as a carbon source by the cells³. The active repressor is not glucose itself but some compound, as yet unidentified, that is derived from it.