Experiment schedule. Four groups of five mice received different immunizations and were subjected to the bites of irradiated or normal infected mosquito bites as indicated. Two groups were immunized by intravenous injection of two doses of irradiated sporozoites two weeks apart (represented by crosses in the figure). After another two weeks, three of the four groups were subjected to two daily bites of infected mosquitoes, irradiated (dashed line) or not (continuous line) for six weeks. One week later (black circles), a small volume of serum was extracted from each mouse to determine anti-sporozoite antibody levels (results shown in Figure 3). Eight weeks later mice were challenged (white circles) and liver stage infection detected with real-time PCR (results shown in Table 1).