Our previous studies have shown that murine αβ TCR+ intraepithelial lymphocytes (IEL) contained T cells that can provide B cell help. In this study, we have examined the three subsets of αβ TCR+ IEL for Ag-specific helper function and cytokine production because αβ TCR+ IEL are divisible into three subsets including CD4+, CD8- T cells, CD4+, CD8+ double positive (DP) T cells, and CD4-, CD8+ T cells. When these three subsets of αβ TCR+ IEL from C3H/HeN mice (H-2(k)) orally immunized with SRBC were cultured with splenic B cells, adherent cells, and SRBC, both CD4+, CD8- and DP T cell fractions supported IgM, IgG1, and IgA anti-SRBC responses, whereas the CD4-, CD8+ T cell subset did not exhibit helper function. Addition of anti-I-A(k) mAb resulted in the reduction of SRBC-specific PFC responses, whereas anti-H-2K(k) mAb did not affect the CD4+, CD8- and DP T cell-supported B cell responses. Furthermore, when these CD4-bearing T cells from mice orally immunized with SRBC were co-cultured with B cells and adherent cells in the presence of unrelated Ag (e.g., horse RBC), SRBC- specific B cell responses were not induced. When type 1 and type 2 Th cell cytokine production was examined by IFN-γ-, IL-2-, IL-4-, and IL-5-specific enzyme-linked immunospot assays, increased numbers of IL-4- and IL-5- secreting type 2 Th cells, whereas lower numbers of IFN-γ and IL-2-producing type 1 Th cells were seen in CD4+, CD8- and DP T cell fractions on in vitro stimulation with SRBC. In the case of CD4-, CD8+ T cells, approximately equal and low numbers of type 2 cytokine-producing cells were noted in both SRBC-stimulated and -unstimulated cultures. Furthermore, when different fractions of IEL T cells from Ag-stimulated and -unstimulated cultures were characterized for cytokine-specific mRNA by reverse transcription polymerase chain reaction, stronger bands for IL-4 and IL-5 were detected in both CD4+, CD8- and DP IEL subsets when compared with the unstimulated cells. In contrast, the change in intensity of the band of polymerase chain reaction product for IFN-γ was slight or unchanged in Ag-stimulated CD4-bearing IEL when compared with unstimulated cells. These results suggest that subsets of αβ TCR+ IEL, and especially the CD4-bearing T cells, are capable of providing helper function for Ag-specific B cell responses. Of significant interest was the finding that DP T cells isolated from the epithelium of the gastrointestinal tract exhibited cytokine synthesis and helper function in addition to CD4+, CD8- IEL expressing a classical helper phenotype.