1. When sporozoite-induced infections of Plasmodium relictum are treated with atebrin (mepacrine) in doses suppressive to blood-inoculated infections, for periods exceeding that of normal incubation, parasites devoid of pigment appear in the erythrocytes of the peripheral blood. A small proportion of these may develop into schizonts, but the number of merozoites produced is fewer than normal.
2. A similar phenomenon occurs when quinine is substituted for atebrin, but the drug appears to be excreted more rapidly so that the inhibitive effect upon growth is less marked.
3. When plasmoquine is given in the maximum tolerated dose to birds infected with sporozoites, parasites are not seen in the peripheral blood during treatment, though the infection is not sterilized. With smaller doses parasites, including a few small schizonts, are found in the erythrocytes during treatment.
4. Merozoites produced in the erythrocytes during drug treatment are not capable of further multiplication when inoculated into uninfected birds having a similar blood-concentration of drug, so long as treatment continues, though they develop normally when transferred to untreated birds.
5. No indication of drug resistance has been observed.
6. The possible mechanisms by which blood-inoculated infections are eradicated, or resist eradication, are discussed.
We wish to thank Miss Vincent (Mrs Thorpe) and Dr Parr Tate for access to their preliminary unpublished experiments, and the latter for helpful criticism.