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What you are testing for will determine the best method for detecting intercellular events. If your target of interest requires spatial resolution, imaging is the best platform to use. The cells will need fixation and permeabilization before labeling. The technical data sheet for the fluorescent product will list this out. If you are looking for a more quantitative approach with a population of cells, flow cytometry is a good choice with its capability for rich statistical information. The cells will need fixation and some assays will require fixation and permeabilization. Of course, live cells assays that use a cell-permeant dye to look at intercellular events do not require cell fixation techniques, for either imaging or flow cytometry platforms.