Rhesus macaques, intravenously inoculated with virulent Rift Valley fever virus, develop viremia and biochemical evidence of liver damage and serve as a model for human disease. Some of these monkeys suffer more serious disease with hemorrhagic phenomena and approximately 20% die with frank hemorrhage. Presently, the only Rift Valley fever vaccine approved for use in humans is a formalin-killed product that requires annual booster vaccinations. Efforts to produce an improved vaccine to replace the present vaccine have led to a mutagen-attenuated strain of Rift Valley fever virus that was found to be markedly attenuated for rhesus macaques and showed promise as a vaccine candidate for human use. Neurovirulence testing in rhesus monkeys showed that, while the vaccine was not completely innocuous, residual lesions were no more severe than the currently used 17D yellow fever vaccine.