Interlaminar fracture (delamination) is the dominant failure mechanism in most advanced composite materials. The resistance of a material system to delamination failure is generally quantified in terms of the interlaminar fracture toughness, Gc. The recently developed Mixed-Mode Bending (MMB) test is unique in that it facilitates measurement of the fracture toughness Gc under any combination of mixed-mode loading from pure Mode I to pure Mode II. This makes it a very attractive technique for developing interlaminar fracture failure criteria. This paper reports on an application of the MMB test to fatigue loading for the generation of Gc versus-curves and identification of the threshold value, Gth. Fatigue results from unidirectional IM7/5260 graphite/bismaleimide and IM7/8320 graphite/thermoplastic specimens are presented for a GI/GII ratio of 0.25. Static test results are also presented for both materials with the mode mix varying from pure Mode I to pure Mode II. The static results indicate different mixed mode failure laws for the two materials. All tests were conducted using displacement controlled loading under room temperature, dry conditions.