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The introduction of microinjection of trace amounts of a fluorescently labeled protein (1,2) into cultured cells leads to its incorporation into the cell's pool of endogenous protein. Provided the microinjected protein has retained the properties of the native protein, it will become incorporated into the same structures as the endogenous protein and will serve, therefore, as a marker of the native protein's distribution in the cell. This allows the changes in a protein's localization to be followed in live cells in response to normal functions such as movement (3) or division (4) or formation of structures like stress fibers or myofibrils (5-9). Responses of the protein to inhibitors of cell function (11,12), or to interactions of the injected cell with other cells such as bacteria, can also be analyzed within a single live cell (13). [ABSTRACT FROM AUTHOR]