An enzyme, beta-D-galactosidase, was covalently coupled to mammalian cells by means of a bifunctional reagent. The coupling procedure did not cause appreciable loss of cell viability (less than 6%) as measured by plating efficiently and membrane integrity. After 24 h in culture, the cells exhibited an average of 2.6 x 10(4) molecules of beta-D-galactosidase per cell. Histological evidence indicated that the enzyme was localized on the cell surface and distributed uniformly among the cell population. Considerations for choosing enzyme-label include sensitivity of assay by enzymatic, immunologic and histochemical methods, and the possibility of isolating labeled membrane components by enzyme-specific affinity chromatography.

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