Qualitative and quantitative changes are well documented for a number of enzyme systems during the dedifferentiation process in neoplasia. The current focus is on mechanisms of derepression and repression which result in alterations in the control of enzyme synthesis. However, intracellular enzyme concentrations are determined by a rate of enzyme synthesis and a rate of enzyme degradation. Alteration of the latter alone could account for the observed changes. We will determine the rate constant of degradation for pyruvate kinase in normal liver and representative Morris hepatomas. Changes in the activity and isozyme type of this enzyme have been shown to be closely related to growth rate for the stable tumor lines in the Morris hepatoma series. We will use the steady state radioactivity method in conjunction With specific immunoprecipitation for the rate constant determination. The values obtained will permit evaluation of the role of degradation in the neoplastic process and a positive interpretation will direct attention to a fundamental aspect of cellular regulation.