Our work deals with aspects of the genetics and immunology of host-parasite relationships as they influence the development of protective immunity and the phenomenon of coevolution. The aim is to understand parasitism through analyses of host specificity. In earlier studies we examined the inheritance of resistances in mice to infections with Nematospiroides dubius (=Heligmosomoides polygyrus) and established the predominant role played by antibodies in protective immunity. Here we report information concerning the selection of lines of N. dubius that differ in their ability to survive antagonistic immunological reactions from mice. Challenge infections with groups of these mice, immunized and protected by previous repeated infections, show that worms selected to survive the immunity that kills other worms do so by inhibiting the effectiveness of the cellular rather than humoral elements of protective immunity.