Hastelloy X and 2/sup 1///sub 4/ Cr-1 Mo steel are being considered as structural alloys for components of a High-Temperature Gas-Cooled Reactor (HTGR) system. Among other mechanical properties, the creep behavior of these materials in HTGR primary coolant helium must be established to form part of the design criteria. This report describes the simulated HTGR-helium environmental creep facilities, summarizes preliminary creep properties of 2/sup 1///sub 4/ Cr-1 Mo steel and Hastelloy X generated in HTGR helium and compares these with data obtained by testing in air. Some corrosion characteristics of the two materials are also discussed.