A study was made of the pneumococcidal action of serum-leucocyte mixtures of pneumococcus-resistant animals with a view to determining whether this property of the blood is to be accounted for by the presence of certain serum constituents or by cellular characteristics which are lacking in the blood of susceptible animals. By means of a method specially developed for this purpose, it was found that, after adequate contact with the serum of pneumococcus-resistant animals, virulent pneumococci were phagocyted actively not only by the homologous leucocytes but also by the leucocytes of other resistant and susceptible animals. On the other hand, pneumococci exposed to the action of the serum of pneumococcus-susceptible animals were not taken up by the leucocytes of either the resistant or susceptible species. All the resistant animals tested, dog, cat, sheep, pig and horse, showed marked opsonic properties in their blood serum which were not found in the serum of susceptible ones, rabbit, guinea pig and human. There appeared, however, to be no essential difference in the phagocytic activity of the leucocytes from the various animals.
It was then shown that the pneumococcus-destroying power of serum-leucocyte mixtures was entirely abolished when heated serum was substituted for fresh serum and that such heated serum had lost much of its opsonic potency. Neither the living leucocytes alone nor extracts of the leucocytes were observed to exert any killing action on pneumococci. Further evidence of the controlling influence of opsonic action in the antipneumococcus defence mechanism of the blood, and its importance in natural resistance, was afforded by a study of the opsonin content and leucocytic functions of the blood of full grown and young rabbits as related to their widely varying degrees of pneumococcus susceptibility.