Micrombules are filamentous protein biopolymers found in eukaryotic cells. They form networks that guide active intracellular transport and support the overall cell structure. Microtubules are very rigid polymers, with persistence lengths as large as a millimeter. As such, they constitute an example of rodlike polymers, whose mechanical and theological properties are as yet poorly understood. We measure the linear and nonlinear viscoelastic properties of isotropic solutions of purified microtubules, as well as networks permanently cross-linked with biotin-NeutrAvidin. In the linear regime both solutions and networks are soft elastic materials with elastic moduli on the order of a few pascals. The elastic moduli show a power-law dependence on tubulin concentration, c(T), with G' similar to c(T)(nu), where v approximate to 1.4 for solutions and increases slightly to nu approximate to 1.6-1.8 for networks. At large deformations, we observe a concentration-dependent yield stress. The rheology of microtubule solutions cannot be explained by the Doi-Edwards model, which treats noninteracting rigid rods. Instead, they show behavior very similar to the permanently cross-linked networks, suggesting the presence of effective cross-linking even in pure microtubule solutions. We develop a simple model based on transient cross-linking interactions between microtubules to interpret the rheological response. We also calculate a lower bound estimate of the strength of this interaction. Our data provide a framework with which to understand the dynamics and mechanics of more physiological networks of microtubules with microtubule-associated cross-linking and motor proteins, and ultimately to understand the role of microtubules in cell mechanics.