The mechanisms of intracellular protein catabolism have not been elucidated in spite of 42 years since Schoenheimer's treatise on "The Dynamic State of Body Constituents". Protein catabolism (cf. Protein Synthesis) involves multiple mechanisms, several of which occur reliably only in cells, not test-tubes. Elucidation of the mechanisms relies on both cell biological and enzymological approaches. Transplantation and microinjection of proteins into target cells, although naturally not without experimental and interpretive difficulties, provide the means of identifying the molecular and cell biological events in intracellular protein degradation. In short, these techniques provide the bioassay system to probe the functions of known catalysts and inhibitors of proteinolysis and provide the means of identifying the currently unknown features of the cytoplasmic surveillance system which present the protein substrates for destruction.