Deformation of the erythrocyte membrane by the micropipette technique permits analysis of intrinsic material characteristics of the membrane and provides a means to differentiate purely membrane factors from such extrinsic factors as surface area-to-volume ratio. Using small micropipettes (less than 0.5 microns radius) to deform cells, it is evident that the red cell membrane behaves like a solid for periods of time up to 5–10 min of sustained deformation; for long periods of strain, permanent deformations occur, indicative of the semi-solid structural character. In the time range in which the membrane behaves like a solid, the material is linearly elastic up to strains of 400%, implying a loose network structure in the membrane plane, and evaluation of the elastic parameter mu (mu for normal discocytes equals 7 x 10(-3) dynes/cm) suggests that the elements comprising the network may have a molecular weight of approximately that of the water-soluble membrane protein spectrin. Whether the network system is cross-linked or simply a polymer solution remains unanswered. Experimental data indicate that plastic flow of the membrane under conditions of protracted strain may lead to permanent deformation of the membrane, whereas uniform dilation of the membrane, requiring over 1000 times more energy than for plastic flow, results in membrane failure and lysis. Analyses of the data from larger micropipettes of limiting mean cylindrical diameter show their utility in evaluating extrinsic factors, e.g., surface area-to-volume relationships, which are related to the capability of the whole cell to form a new configuration with implicit resistance to total surface area change, as the cell enters narrow channels of the microcirculation. Thus, micropipettes with diameters in the 2.7–3.0-microns range can provide sensitive comparisons of cellular deformability of erythrocytes.