Abstract As part of a dengue vaccine study, it was necessary to determine how much virus was required to infect rhesus monkeys. Serial dilutions of dengue 2 and 4 viruses were inoculated subcutaneously into groups of five monkeys and seroconversions determined on days 30 and 60 post-inoculation. The viruses were also titrated simultaneously in LLC-MK 2 cells and mosquitoes. It was calculated that 9·5 mosquito infectious doses 50 (MID 50) of dengue 2 virus and 22 MID 50 of dengue 4 virus were required to infect 50% of the monkeys. The data suggest that 100 MID 50 of dengue virus should be used as a challenge dose for monkeys previously immunized with dengue vaccine.