Red blood cell (RBC) shape change under shear is generally reversible, with the time course of shape recovery a function of the elastic and viscous properties of the RBC membrane. RBC shape recovery can be investigated, using several different techniques, to provide information about the membrane material properties that are not directly accessible by frequently used methods to assess RBC deformability (e.g., micropore filtration). In the present study, RBC shape recovery was studied in a Couette system after abrupt cessation of shear, either by analyzing the time course of laser light reflection or by serial measurements of elongation indexes from laser diffraction patterns. The time course of shape recovery monitored with both techniques can be described with an exponential equation. Calculated time constants for normal human RBC were 119 +/- 17 msec and 97 +/- 15 msec as measured by light reflection and ektacytometry, respectively. Treatment of RBC with glutaraldehyde resulted in dose-dependent decreases in the shape recovery time constant. Heat treatment (48 degrees C, 20 min), which is known to increase mainly the shear elastic modulus of the membrane, decreased the time constant by 65%. In contrast, wheat germ agglutinin treatment increased the shape recovery time constant by 22%, presumably by increasing membrane surface viscosity. Our results indicate that the shape recovery time constant of RBC can be measured easily and accurately by computerized light reflection analysis.