SummaryExperiments are described in which preparations of yellow fever virus, Bwamba fever virus and influenza virus were allowed to react with corresponding antisera labeled with radioactive iodine (I131). No significant differences were observed in the activity of the sediments obtained with the first 2 virus systems from those of appropriate controls. The influenza virus-immune serum sediment showed significantly more activity than any of its control preparations. It is suggested that the technic may be of value in virus studies where relatively pure and concentrated virus preparations are available.