The pattern of mortality for P. berghei infection of mice has been described variously as including early and late peaks (W. Eling et al., 1977, Z. Parasitenkd. 54: 29-45). Early deaths occur at the end of the 1st wk of infection, sometimes before the onset of anemia. The immediate cause of death is unknown, although the pattern of mortality does depend strongly on the strain of both the host and parasite. Preliminary experiments in Balb/c, CBA, and A/J mice suggested to us that an additional influence was the immediate provenance of the parasite. If the parasite had been previously passaged in mice of the same strain, the early death rate was much lower than it was when it had been passaged in different strains. This was confirmed in the following experiments. The 12th day of infection was chosen for analysis because preliminary results suggested that survival differences were most marked at 10 to 14 days. Balb/c mice were injected i.p. with 105 P. berghei-infected red cells obtained from Balb/ c mice after four consecutive passages within that strain. Only 7% (1/15) of the mice were dead by day 12. However, the same number of infected cells obtained from CBA mice killed 40% (6/15) of the Balb/c mice by day 12. The time to 100% mortality was the same in both groups. Simultaneous experiments with CBA mice injected with parasites passed in CBA strains showed no deaths by day 12. The parasites obtained from Balb/c mice killed 47% (8/17) of the CBA mice. The difference within each pair was shown to be significant (P < 0.01) using the Kaplan Meier test to compare the survival curves. We next injected mice with homologous, uninfected erythrocytes (106) in addition to the infecting dose of autologous (uninfected) red cells to see if these foreign antigens themselves would affect the pattern of mortality. They did not. Our findings suggested the early mortality may be related to the use of parasites passed in a different strain and are consistent with some of the results of Greenberg and Kendrick (1957, J. Parasitol. 43: 420-428). They showed that inoculation of P. berghei previously passaged in Swiss mice produced a significantly higher mortality in Balb/c, C3H, and other strains than did parasites previously passaged within these strains. Generally, this was unrelated to a difference in levels of parasitemia. They found, however, that parasites from C3H mice were less rapidly lethal for Swiss mice than those previously passaged with the Swiss strain. This latter result does not agree with our findings, but the authors had already shown that the Swiss to C3H combination was rather unusual in its behavior, and no other examples of this experiment (i.e., parasites passaged in other strains and injected into Swiss mice) were studied. It seems possible that not all strain combinations behave the same way. In initial experiments, we have found that although parasites passaged in A/J mice seem to be more lethal to CBA than those from CBA mice, the reverse experiment showed no differences in mortality. This finding that the characteristics of the parasite or the host response may well be altered by previous passage of the parasite