A single kind of red cell is supposed to have an enormous number of different substances on it, and in the same way there are substances in the serum to react with many different animal cells. In addition, the substances which match each kind of cell are different in each kind of serum. The number of hypothetical different substances postulated makes this conception so uneconomical that the question must be asked whether it is the only one possible. ... We ourselves hold that another, simpler, explanation is possible.