The discovery of exoerythrocytic schizogony in certain avian malarias led very early to attempts at cultivation of the exoerythrocytic parasites in vitro. As early as 1935, in their study of the schizogony of Plasmodium elongatum, Huff and Bloom tried to grow the parasites in tissue cultures of the bone marrow of infected canaries. Parasites were not observed microscopically in these cultures, but evidence was obtained that they survived at least for short periods. By inoculation into normal birds the cultures were shown to contain viable parasites after 48 hours. Survival for such a short time would appear insignificant were it not for the fact that when cultures were made from tissues of birds infected with both P. elongatum and P. cathemerium, only the former was recovered after 48 hours. While the report does not demonstrate in vitro growth, it does indicate survival under conditions unfavorable to erythrocytic stages. This study appears to be the only one reported on attempted cultivation of the elongatum-type exoerythrocytic schizonts, which develop in all cells of the erythropoietic series as well as in cells of the lymphoid-macrophage system. With the discovery of gallinaceum-type exoerythrocytic stages in P. relictum by Raffaele (1936), in P. gallinaceum by James and Tate (1937), and later in P. cathemerium, P. circumflexum and P. lophurae, efforts were instituted to demonstrate development of these parasites in connective tissue cells grown in vitro. In these species the exoerythrocytic stages develop almost exclusively in macrophages and capillary endothelium and are morphologically distinct from those reported in P. elolngatum. This statement must be qualified with mention of the fact that in one individual chick embryo infected with P. gallinaceum Zuckerman (1946) found exoerythrocytic stages of the elongatum type. Since macrophages grow readily in tissue culture, it appeared possible that these gallinaceum-type exoerythrocytic stages could be cultivated in vitro. The first attempts were those of Gavrilov, Bobkoff and Laurencin (1938). They cultivated fragments of tissue from chicks infected with P. gallinaceum in plasma and embryo extract. Cultures of bone marrow were incubated for seven days and then injected subcutaneously into a normal chick. The test bird became infected, with a prepatent period of eight days. In a similar fashion chicks were infected by intravenous and intramuscular inoculation of bone marrow cultivated for ten days. The long prepatent period, 11 days, indicated, however, that few parasites were present in the culture. Hanging-drop cultures of bone marrow were also infective after ten days, but efforts at subculture were unsuccessful. The authors divided and replanted cultures of the same series but obtained no evidence of persistence of infection. Normal bone marrow cultivated along-side the infected tissues did not become parasitized. Maitland cultures of surviving tissues (minced embryo and infected 300