Crack initiation and propagation were studied in three tritium-exposed stainless steels. The purpose was to measure cracking thresholds and velocities as a function of helium concentration in Type 21-6-9 stainless steel and compare the results to earlier measurements on Types 316L and 304L steels. Fracture toughness specimens were cut from forgings, fatigue-cracked and exposed to tritium at 423 K and 31 MPa. The samples were aged for selected times at 273 K to “build-in” 3He from tritium decay. Tritium concentrations ranged from 0-2600 atomic parts-per-million (appm) and 3He concentrations ranged from 0-600 appm. The samples were step-loaded at room temperature in air using a screw-driven mechanical testing machine and held at fixed displacement until crack initiation was detected. Crack propagation was monitored by continuously recording the drop in load until crack arrest. Threshold stress intensity was calculated from the load and the crack length at the end of the test. Crack velocities were determined from the load-time records and compliance relationships and verified on some samples using a DC potential-drop technique. The crack path was along grain and twin boundaries. For 21-6-9, the threshold for cracking decreased with increasing helium concentrations from about 90 MPa-m1/2 (50 appm helium) to 25 MPa-m1/2 (600 appm helium). Steady-state-crack velocities averaged 10-7 m/s and was not strongly dependent on helium concentration. The data show that embrittlement of tritium-exposed stainless steels is a form of hydrogen embrittlement made worse by the hardening of the microstructure from nanometer-sized helium bubbles that build-in with tritium decay.