P. malariae was first used for therapy purposes at Horton in 1928.

Between 1928 and 1945, nine strains have been used. Most of these were from Africa, but some were from Holland, Roumania and South America. The strain from Holland was probably not indigenous to that country.

All transmission experiments carried out between 1928 and 1945 were with Anopheles maculipennis var. atroparvus. Nineteen separate batches of mosquitoes were fed on patients in whose blood parasites believed to be gametocytes were seen. In only six of these batches were any of the mosquitoes found to be infected and then with only scanty oocysts; sporozoites were never found. No patients were infected by the bites of any of these mosquitoes.

In 1947 a colony of A. stephensi (type) was established.

From 1948 to 1950, sixty-eight batches of A. stephensi were fed on patients in whose blood Plasmodium malariae parasites were present. Scanty gametocytes were found in some of these patients, and on occasion mosquitoes which fed on them became infected, but in others no infection occurred.

Other batches of mosquitoes did become infected after feeding on patients in whose blood no typical gametocytes were found even after a prolonged search. Out of sixty-eight batches of mosquitoes fed, infection was seen in twelve batches, in varying degree, but never with more than nine oocysts in an individual mosquito. Two consecutive human anopheline passages with laboratory-bred Anopheles mosquitoes (A. stephehsi type) and induced human carriers are reported.