Abstract 1. 1.| Meso-tartrate-grown pseudomonads rapidly lost the induced enzyme, meso-tartrate dehydratase, in the absence of the inducer. The rate of enzyme loss was similar in washed cells resuspended without a carbon source and in cells continuing to grow in succinate or glucose media. Azide or dinitrophenol did not affect the rate of enzyme disappearance. The level of another inducible enzyme, histidine ammonialyase, did not drop under these conditions. 2. 2.|Enzyme destruction in whole cells was markedly dependent on temperature, being 7% per 30 min at 25° and almost 10 times as fast at 37°. Temperatures of 25° or higher also inactivated the enzyme in extracts. In extracts, addition of tartrate isomers or incubation under N 2 stabilized the dehydratase, whereas glucose or succinate had no effect. These findings could be most readily explained by assuming that meso-tartrade dehydratase is a heat-sensitive protein which can be protected against denaturation in vivo by the inducer and its analogues or by incubation under N 2.