1. 5-Phosphorylribose 1-pyrophosphate, in the presence of β-mercaptoethanol, protected β-galactosidase from heat inactivation. Many other substances, including 3′:5′-cyclic-AMP, were without effect. 2. The efficiency of complementation in vitro of β-galactosidase segments was decreased by 5-phosphorylribose 1-pyrophosphate but not by 3′:5′-cyclic-AMP. Neither substance affected the activity of the complete enzyme. 3. Some indications as to the possible identity of the catabolite repression effector are presented.
