1. The development of exo-erythrocytic parasites of Plasmodium gallinaceum is more rapid and intense in chicks inoculated with blood from birds heavily infected with exo-erythrocytic parasites than in those inoculated with blood from birds in which no exo-erythrocytic parasites can be found.
2. If parasitized blood from a bird heavily infected with exo-erythrocytic parasites is inoculated intravenously into a normal bird and withdrawn 24 hr. later, it produces infections in normal birds characterized by a slow development of exo-erythrocytic parasites.
3. The proportion of birds which survived after inoculation with P. gallinaceum and daily treatment with suppressive doses of quinine, mepacrine or pamaquin, was greater when the inoculum was derived from donors in which no exo-erythrocytic parasites could be found (48%) than in those inoculated with blood from donors in which exo-erythrocytic parasites 'were numerous (15). No birds inoculated with parasitized blood from donors heavily infected with exo-erythrocytic parasites were sterilized of their infections by the action of the drugs, whereas in eleven out of sixty-six of the birds inoculated with parasitized blood from birds in which no exo-erythrocytic parasites could be found the infections were eradicated by the drugs.
4. Blood-inoculated infections of P. gallinaceum may in some cases be eradicated by short courses of treatment with mepacrine, quinine or pamaquin.
5. Whereas blood-inoculated infections of P. gallinaceum derived from donors in which no exo-erythrocytic parasites could be found were sometimes eradicated by prophylactic treatment with quinine, mepacrine or pamaquin, it was not possible to eradicate the infections by curative treatment with similar doses of these drugs.
6. The difference in type of infection produced by parasitized blood from birds in which no exo-erythrocytic parasites could be found at the time of withdrawal of the blood and those in which exo-erythrocytic parasites were plentiful appears to be due to the presence of exo-erythrocytic parasites in the circulating blood of the latter type of donor.
7. Sterilization by the action of quinine, mepa-crine or pamaquin of blood-inoculated infections from donors in which no exo-erythrocytic forms could be found, appears to depend upon whether the erythrocytic parasites are killed before they can become converted into exo-erythrocytic forms.
8. No evidence has been obtained of the intensification of development of exo-erythrocytic parasites by the suppression of erythrocytic parasites with drugs.