Mice of 6 different F1 generation hybrids; Swiss x A/LN, Swiss x DBA/2JN, Swiss x C3H/Hen, Swiss x C57B1/6JN, Swiss x BALB/CANN, STR/N x C57B1/6JN, and general purpose Swiss, were inoculated with Plasmodium berghei. The 6th-day parasitemia of mature erythrocytes as well as survival was observed. Swiss x A, Swiss x DBA, Swiss x C3H and Swiss x BALB formed a common group having higher parasitemia than Swiss x C57B1 or STR x C57B1 but significantly lower than Swiss mice. Swiss x C57B1 mice had the lowest parasitemia of any Swiss hybrids observed. STR x C57B1 mice demonstrated a significantly lower parasitemia than any Swiss hybrid with the possible exception of Swiss x C57B1. Survival in the hybrids, with the exception of Swiss x DBA and Swiss x C57B1, demonstrated a close similarity to that of the longer lived parent strain. Thus. STR x C57B1 was the longest lived hybrid considered. Survival of Swiss x DBA was significantly longer than that of either parent strain while that of Swiss x C57B1 demonstrated a closer similarity to the shorter lived, rather than the longer lived parent strain.