obvious properties, its structure, composition, electrical properties, me­ chanical properties, and most particularly, its permeability properties. None of these properties are obviously associated with surface enzymes. What are the functions of the cell surface that may require the presence of enzymes? From a purely hypothetical point of view, the cellular activities in which cell surface enzymes are potentially functional can be divided into two categories; first, those activities related to the production and maintenance of the cell surface structure itself, and second, those activities which are concerned with interactions between the cell and its environment. During growth of a cell, as it enlarges, new structural ma­ terial must be synthesized and assembled in the cell membrane. Although it is conceivable that new structural material is synthesized in the interior of the cell, it is difficult to visualize its complete synthesis and final assembly in the periphery of the cell, without the assistance of enzymes at the site. In the case of non-growing cells, although no net synthesis of new structural material is necessary, the turnover of the various components by a continuous degradation and resynthesis must be considered as a pos­ sibility.