The American Commission obtained ova from Doctor Finlay and from these ova mosquitoes were hatched for the experimental work. Of 11 susceptible persons, bitten by contaminated mosquitoes, the first 9 remained uninfected while the two volunteers, bitten subsequently (Aug. 27 and 31), developed the disease. One of these cases was that of Doctor Carroll, whose infection was brought about by one of these laboratory reared mosquitoes which had fed on 4 cases of yellow fever, two of which were severe cases and two mild. This mosquito had fed on one of the severe cases just twelve days previously. The other case was bitten by 4 contaminated mosquitoes, one of which was the one that had infected Doctor Carroll. Of the 9 negative cases 6 were bitten by mosquitoes which had fed on yellow fever patients from the fifth to the seventh day of the disease and the remaining 3 failures were where the interval between contamination and biting the volunteer was from two to six days only. At this time the medical mind was obsessed with the idea that yellow fever was transmitted by fomites. It had been forgotten that Cathrall, in 1800, had failed to infect himself with black vomit and that Ffirth, in 1804, in order to obtain material for a graduation thesis, swallowed black vomit and smeared it, as well as blood, upon wounds he had made on his skin with negative result. (It will be remembered that the fourth proposition of the French Commission was that application of infectious serum to the abraded skin would not produce the disease—the hypodermic injection being required.) =Fomites.=—To settle the question of the relative importance of fomites and infected mosquitoes the Commission caused two houses, 14 × 20 feet, to be erected, one well ventilated for the infected mosquitoes and the other poorly lighted and ill ventilated for the black vomit contaminated clothes, sheets and blankets (fomites). A medical officer and two privates of the Hospital Corps slept in this room for twenty days in most intimate contact with the infected material. No infections resulted. There were other experiments with similar results. One of the occupants of the fomites building was afterward inoculated subcutaneously with 2 cc. of blood taken from a patient in the first day of the disease and developed yellow fever after four days of incubation. This proved that he was susceptible to yellow fever. Blood from this patient, taken in the first three days of his attack, was injected into a third man who also developed yellow fever. This experiment was to prove that the production of the disease was due to a virus capable of multiplying rather than to a toxin. Of course, it would be impossible to conceive of a toxin so potent that it could produce symptoms in a third man when diluted in the circulation of the second man.