Although investigation into the structure of eukaryotic cell membranes has been an intense focus of cell biology for the past two decades, definitive insights have been limited by the lack of coherent methods for the isolation of specific organelle membranes and the identification of membrane subdomains. Here we describe a method for the isolation of mammalian cell plasma membranes as Giant Plasma Membrane Vesicles (GPMVs) and strategies for imaging membrane lateral structure and quantification of protein partitioning between coexisting domains by fluorescence microscopy.