The study describes the development and characterization of a monoclonal antibody (mAb) named 145-2C11, which is specific for the murine T3 complex. This complex, involved in T-cell activation, consists of several low molecular mass proteins, including T3-δ, T3-γ, and T3-ε. The mAb was generated by immunizing Armenian hamsters with a murine cytolytic T-cell clone and was identified through a screening assay designed to detect mAbs directed at cell surface molecules involved in T-cell activation. The 145-2C11 mAb reacts with a 25-kDa protein (T3-ε) that is noncovalently associated with the antigen-specific αβ heterodimer of the T-cell receptor (TCR). Functional analyses revealed that the anti-murine T3-ε mAb can both activate and inhibit T-cell function, indicating its role in transducing activation signals. This mAb will be useful for studying the role of various components of the TCR complex in T-cell activation and development, as well as for investigating the immunoregulatory effects of in vivo administration of anti-T3 antibodies.The study describes the development and characterization of a monoclonal antibody (mAb) named 145-2C11, which is specific for the murine T3 complex. This complex, involved in T-cell activation, consists of several low molecular mass proteins, including T3-δ, T3-γ, and T3-ε. The mAb was generated by immunizing Armenian hamsters with a murine cytolytic T-cell clone and was identified through a screening assay designed to detect mAbs directed at cell surface molecules involved in T-cell activation. The 145-2C11 mAb reacts with a 25-kDa protein (T3-ε) that is noncovalently associated with the antigen-specific αβ heterodimer of the T-cell receptor (TCR). Functional analyses revealed that the anti-murine T3-ε mAb can both activate and inhibit T-cell function, indicating its role in transducing activation signals. This mAb will be useful for studying the role of various components of the TCR complex in T-cell activation and development, as well as for investigating the immunoregulatory effects of in vivo administration of anti-T3 antibodies.