T cells require CD28/CTLA-4 costimulatory molecule interactions in addition to Ag-specific signals through the TCR for in vivo effector Th cell function. Some studies have suggested that the ligands for these costimulatory molecules may differentially influence effector T cell function with B7-2 favoring a type 2 response and B7-1 favoring a type 1 response, while other studies have suggested that these molecules may be redundant. The recent development of B7-2-deficient mice permits the direct analysis of the requirement of B7-2 during a type 2 immune response to an infectious pathogen. We have examined, in B7-2-deficient mice, effector Th cell function and the associated type 2 immune response following infection with Heligmosomoides polygyrus, a natural murine parasitic nematode. Elevations in cytokine gene expression and protein secretion were pronounced and comparable in inoculated B7-2-/- and B7-2+/+ mice at day 8 after H. polygyrus inoculation. However, by day 14 after infection, increases in T cell cytokine expression were markedly inhibited in H. polygyrus-inoculated B7-2-/- mice. Furthermore, elevations in serum IgE and germinal center formation were inhibited at later stages of the immune response, while elevations in serum IgG1 persisted. These findings suggest that certain T-dependent components vary in their B7-2-dependency during the type 2 immune response. They further demonstrate that B7-2 interactions are not necessary for the initiation of the type 2 immune response, but are instead required for its progression after the development of effector T cells.