That the serum of a pigeon does not differ in its action upon pneumococci from the serum of a mouse or rabbit would appear from our experiments as well as from those of previous investigators.

We have found, however, that if small numbers of living pneumococci are seeded, by a suitable method, in pigeon's blood before it coagulates the penumococci fail to multiply. On the contrary if pneumococci are seeded in mouse's or rabbit's blood before it coagulates the pneumococci grow with great vigor. The evidence points to the presence in the uncoagulated blood of the pigeon of a bactericidal factor which is absent from the blood of the mouse or rabbit.

Fresh defibrinated blood of both immune and susceptible species is an excellent culture medium for the pneumococcus, which shows that the bactericidal factor disappears during the process of coagulation.

Pneumococci fail to multiply in the blood of a rabbit which has been suitably inoculated with killed pneumococci, the reaction being specific to type.

The blood of a man that has recovered from lobar pneumonia prevents the growth of pneumococci belonging to the type which caused his disease.

The globoid bodies of poliomyelitis grow readily in uncoagulated human blood. They fail to grow when seeded in uncoagulated rabbit blood.

Many of the non-pathogenic bacteria usually met with as contaminations in bacteriological work, as for instance the B. subtilis, fail to grow when seeded in uncoagulated blood.

The tentative hypothesis advanced to cover the facts observed is that when small numbers of bacteria are seeded in blood before it has had time to coagulate only those bacteria grow and multiply which are pathogenic for the animal from which the blood is taken.

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