The nonfixable polar tracer, 5-(and-6)-carboxy-2',7' -dichlorofluorescein, can be used to investigate cell-cell and cell-liposome fusion as well as membrane permeability and transport through gap junctions or cell uptake during pinocytosis. This water-soluble tracer can be introduced into cells by whole-cell patch clamping, iontophoresis, osmotic lysis of pinocytic vesicles or comparable methods. With a pK_a of 4.8, this can also be used as an acidic pH sensor.