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The white mice did not go along for the ride. They went to prove which type of pneumonia was responsible. Mice, to qualify for this work, must be bred through many generations. Their family and life history must be known in order to avoid introducing complicating factors. Pedigreed Canadian mice would have done as well, but since the plane and the serum were going anyway, there was no need to take a chance on not finding the mice.
Floyd Bennett had been in the hospital only one day when the plane left New York. It is not always possible to determine the type of pneumonia in that time. Unfortunately he had Type III double lobar pneumonia, the most dangerous of all the pneumonias, for which no reliable serum has as yet been developed. Had it been Type I or II, the press might now be full of encomiums for the quick thinking, quick acting expedition that saved his life.