This study demonstrates that ingested host is involved in schistosome nutrition. Homologous reticulocytes, labeled with tritiated L-leucine, were injected into infected mice. Four days postinjection the animals were killed and the schistosomes were recovered by perfusion. The harvested worms were examined for radioactivity by direct counting and radioautography. These experiments clearly demonstrate that L-leucine was incorporated and extensively distributed in the tissues of the schistosomes. Previous investigators have concluded that ingested is involved in the amino acid nutrition of schistosomes because material resembling hematin has been observed in the guts of worms (Rogers, 1940) and because an enzyme associated with the worm, hemoglobin protease, specifically hydrolyzes host (Timms and Bueding, 1959). However, these conclusions are based upon in vitro experiments and microscopic observation of harvested worms; there has been no direct experimental data to support the hypothesis that ingested blood cells are utilized by the parasite. At the outset, in vitro experiments seemed attractive to us because of their relative simplicity and because it has been possible to maintain schistosomes for at least 60 days in artificial media (Ross and Bueding, 1950; Robinson, 1954; Senft and Senft, 1962; and our unpublished experiments), a period entirely satisfactory for the labeling of worms for a radioautographic study. However, in our experience, worms maintained in artificial medium did not consistently evacuate their gut contents nor did they seem to ingest particulate material from the medium, an obstacle in getting radioactive blood cells into the schistosome cecum. We showed that the failure of worms to evacuate their guts in vitro involved more than a lack of stimulation by particulate matter because when latex spheres, approximately the size of red blood cells (8 yt), were presented to engorged worms in artificial medium, neither was the gut evacuated nor were the spheres ingested. Also, we observed that worms incubated in tissue culture medium 199 (Morgan et al., 1950) with 10% serum, or in Received for publication 17 June 1969. the schistosome medium cited above, failed to maintain normal oogenesis and that the size of the mature worms decreased with prolonged incubation even though the medium was changed periodically. This suggests that these media do not completely satisfy the nutritional requirements of the worms. Finally, we felt that the most directly interpretable data would be provided by in vivo experiments and that, at any event, in vitro results would have to be supported later by in vivo studies. Thus we decided to proceed directly with in vivo experiments. The experiments reported herein were designed to ascertain whether or not S. mansoni utilizes host for the synthesis of tissue components. MATERIALS AND METHODS 1. Production of reticulocytosis Ten CFP female mice were injected subcutaneously, once daily for 12 days, with 0.1 ml of 0.25% phenylhydrazine hydrochloride solution, neutralized to pH 7.0 with sodium hydroxide. Blood samples were removed by infraorbital sinus puncture. Thin blood smears were treated with methylene blue reticulocyte reagent and the percentage of reticulocytes was determined by direct