We have analyzed the requirement for β 2-microglobulin (β 2 m)-dependent T cells in the generation of allogeneic Th2 responses in vivo. A neonatal injection of semiallogeneic cells in BALB/c mice induces a state of chimerism that promotes the differentiation of donor-specific CD4+ T cells toward the Th2 phenotype. Polyclonal TB cell interactions occur in this model between host Th2 and donor B cells, resulting in the production of IgE Abs. IgE production and Th2-priming are critically dependent upon the early production of IL-4. Our data in the present paper demonstrate that: 1) IgE synthesis and the up-regulation of MHC class II and CD23 molecules on B cells are independent of β 2 m expression in the host, 2) no difference in the induction of CD4 alloreactive Th2 cells could be observed between β 2 m−/− and their wild-type control littermates when Th2-priming was measured in adult mice, and 3) the Th2 response and IgE production is induced in the complete absence of β 2 m-dependent T cells both in the host and in the inoculum. Therefore, using a variety of assays, we could not demonstrate diminished responses in mice with a disrupted β 2 m gene in this model of Th2-mediated allogeneic interaction, indicating that β 2 m-dependent NK1. 1+ and CD8+ T cells are not required for the generation of alloreactive Th2 responses in vivo.