Giant Plasma Membrane Vesicles (GPMVs) are a widely used model system for biochemical and biophysical analysis of the isolated mammalian plasma membrane (PM). A core advantage of these vesicles is that they maintain the native lipid and protein diversity of the plasma membrane while affording the experimental flexibility of synthetic giant vesicles. In addition to fundamental investigations of PM structure and composition, GPMVs have been used to evaluate the binding of proteins and small molecules to cell-derived membranes, and the permeation of drug-like molecules through them. An important assumption of such experiments is that GPMVs are sealed; i.e. that permeation occurs by diffusion through the hydrophobic core rather than through hydrophilic pores. Here we demonstrate that this assumption is often incorrect. We find that most GPMVs isolated using standard preparations are passively permeable to various hydrophilic solutes as large as 40 kDa, in contrast to synthetic giant unilamellar vesicles (GUVs). We attribute this leakiness to relatively large and heterogeneous pores formed by rupture of vesicles from cells. These pores are stable and persist throughout experimentally relevant time scales. Finally, we identify preparation conditions that minimize poration and allow evaluation of sealed GPMVs. These unexpected observations of GPMV poration are of critical importance for interpreting experiments utilizing GPMVs as plasma membrane models, particularly for drug permeation and membrane asymmetry. STATEMENT OF SIGNIFICANCE:A critical assumption in using Giant Plasma Membrane Vesicles to study membrane penetration and interactions is that these vesicles maintain the permeability barrier of the native membrane from which they form. Using large fluorescently-labeled hydrophilic probes, we demonstrate that this assumption is often incorrect and conclude that macromolecular solutes permeate GPMVs through stable pores formed during shear-induced rupture of vesicles from cells. Using these insights into the mechanisms of poration, we demonstrate an approach to isolate sealed GPMVs.