Cultures of normally proliferating Hela cells have been examined in thin sections by electron microscopy following glutaraldehyde fixation, staining in Wachstein and Meisel's adenosine triphosphate containing medium, postosmication, and embedding in an epoxy resin. The cells were stained in suspension in order to ensure uniform accessibility to reagents. Discrete localization of the enzyme reaction product (lead phosphate) was found at the plasma membranes of about half the cells, but nowhere else. It appeared in the form of an intensely electron-opaque deposit lying close against the outer surface of the cells and varying in amount from a chain of small particles to a dense band about 30 mµ in width. This opaque reaction product was present over microvilli when absent elsewhere on a cell, was heaviest where microvilli and processes were profuse, and was minimal or lacking where cell surfaces were smooth. These observations are discussed in relation to both the idea that surface enzyme activity varies with the physiological phase of individual cells in a population, and the problem of how such enzyme activity becomes manifest at a given site on a morphologically changing membrane system.

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