A high-velocity stream of nitrogen is used to simultaneously disrupt myocardial cells in monolayer culture and fractionate their sarcolemmal membranes. The membranes show a high degree of ultrastructural and enzymatic purity, with less than 1 percent intracellular residuum. They are produced in less than 1 second and remain as tightly adherent sheets on the surface on which the cells were grown. The cells are exposed to no agent other than nitrogen gas during the preparative procedure.