To determine the specific contribution of cytoskeletal proteins to cellular viscoelasticity we performed rheological experiments with Dictyostelium discoideum wild-type cells (AX2) and mutant cells altered by homologous recombination to lack alpha-actinin (AHR), the ABP120 gelation factor (GHR), or both of these F-actin cross-linking proteins (AGHR). Oscillatory and steady flow measurements of Dictyostelium wild-type cells in a torsion pendulum showed that there is a large elastic component to the viscoelasticity of the cell pellet. Quantitative rheological measurements were performed with an electronic plate-and-cone rheometer, which allowed determination of G', the storage shear modulus, and G", the viscous loss modulus, as a function of time, frequency, and strain, respectively. Whole cell viscoelasticity depends strongly on all three parameters, and comparison of wild-type and mutant strains under identical conditions generally produced significant differences. Especially stress relaxation experiments consistently revealed a clear difference between cells that lacked alpha-actinin as compared with wild-type cells or transformants without ABP120 gelation factor, indicating that alpha-actinin plays an important role in cell elasticity. Direct observation of cells undergoing shear deformation was done by incorporating a small number of AX2 cells expressing the green fluorescent protein of Aequorea victoria and visualizing the strained cell pellet by fluorescence and phase contrast microscopy. These observations confirmed that the shear strain imposed by the rheometer does not injure the cells and that the viscoelastic response of the cell pellet is due to deformation of individual cells.