A nonhuman primate model of Rocky Mountain spotted fever infection was developed in cynomolgus monkeys (Macaca fascicularis) infected by the subcutaneous route or by aerosol. Clinical responses, hematology and serum chemistry values, and pathological findings were similar to those found in humans ill with Rocky Mountain spotted fever. The clinical model was then used to test the efficacy of a killed Rocky Mountain spotted fever vaccine grown in chicken embryo cells. Monkeys were immunized with varying dilutions of the vaccine with a two-dose schedule and then challenged at 2 months with virulent Rickettsia rickettsii by the subcutaneous route or by aerosol. The undiluted vaccine totally protected monkeys against both challenges, even at extremely high doses.