Red cells moving in a stream of fluid can be arrested by means of a network of fibrin threads. Those cells which fold over a fibrin thread present to view a flat strap-like portion of membrane in which the strain can be easily observed and accurately measured both optically and by scanning electron microscopy. Preliminary results obtained by this method show elastic strains of nearly 300% and indicate that the stress required to produce these large strains is an order of magnitude greater than that reported by other methods.