Clinical and serological responses investigated in rhesus and cynomolgus monkeys infected with either Bunyamwera or Germiston viruses were generally minimal or absent. A febrile reaction occurred in one of eight monkeys, and another monkey died unexplainably among those given Bunyamwera virus. All monkeys infected with Germiston virus by subcutaneous, intravenous, or intracerebral routes developed fevers, and all infections were relatively consistent in time of onset, duration, and level of viremia. Rhesus monkeys were also susceptible to infection by exposure to aerosolized Germiston virus, although viremias were irregular in these animals, and no fevers were detected. Serological responses to infections by Bunyamwera and Germiston viruses (hemagglutination inhibition, complement fixation, and neutralization tests) provided more reliable indications of infection than clinical response.