Mechanical properties of TiAl base alloys were studied with three-point bending tests in vacuum, air, and gaseous hydrogen. The TiAl-Mn and TiAl (50:50) alloys homogenized at 1273 K exhibited relatively good bend ductility (about 2.5-3.0%) in vacuum, and poor ductility (about 1%) in air and hydrogen. The Ti-rich and Al-rich TiAl alloys, which exhibited little ductility even in vacuum, hardly showed any environmental effect on ductility. These facts may suggest that the mechanical properties of TiAl base alloys are effected at room temperature by the laboratory air and hydrogen, and that plastic deformation plays an important role in environmental embrittlement of TiAl base alloys at room temperature.