A population of CD4+ T cells with TCR beta-chain without TCR alpha-chain (CD4+, betabeta T cells) producing Th2-type cytokines increased in the mucosal and peripheral tissues of TCR alpha-chain deficient mice with inflammatory bowel disease (IBD). Analysis of TCR-beta immunoprecipitates by two-dimensional electrophoresis and RT-PCR revealed TCR of the CD4+ T cells was a homodimer of TCR beta-chains. Polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analyses of TCR Vbeta-chain transcripts of the betabeta T cells revealed monoclonal to oligoclonal accumulation of the cells in the colon, suggesting clonal expansion of the mucosal betabeta T cells upon the stimulation with gut-derived antigens. The homodimer of TCR beta-chains on the betabeta T cells was a biologically functional receptor that transduced activation signals provided by MHC-class II-associated peptidic antigens and superantigens. Treatments of the mutant mice with mAb against TCR beta or IL-4 suppressed the onset of IBD. These findings suggest that the generation of oligoclonal Th2-type betabeta T cells plays a critical role for the development of IBD.