1. Animals susceptible and naturally immune to plague—Bombay rats and white mice—were infected with Past, pestis and supravital study of white blood cells from the peripheral blood stream carried out. Similar studies were made on white mice injected with (a) pure envelope serum, (b) pure somatic serum, and (c) whole antiplague serum.
2. Different experimental conditions produced different cell pictures with different behaviours of individual cell types, especially the polymorpho-nuclears, the monocytes and the clasmatocytes. These abnormalities were found to bear distinctive relationship to (a) active immunity, (b) passive immunity, and (c) susceptibility on the part of the experimental animal.
3. The value of immunological inferences from this study in relation to plague-serum therapy and plague prophylaxis has been emphasized.
4. The possibility of a better understanding of host-parasite relationships from similar studies in other bacterial infections has been pointed out.
Our thanks are due to Lt.-Col. S. S. Sokhey, I.M.S., Director Haffkine Institute, for his interest in this investigation.