Tensile and creep-rupture data have been obtained at 700 and 760°C for several experimental heats of lncoloy 800 that were irradiated in the Oak Ridge Research Reactor (ORR) at elevated temperatures or held at control temperatures in the laboratory. Effects of composition, grain size, and stress level (or strain rate) on these strength properties were investigated. Enhanced postirradiation ductility was observed in Incoloy 800 containing ∼0.1% Ti in creep-rupture tests. The maximum ductility for this composition was obtained in fine-grained specimens tested at low creep stress levels, and the ductility appeared to be unaffected by carbon content. The ductility of unirradiated material was also increased by decreasing either grain size or strain rate. Significant variations in properties (both control and postirradiation tests) were noted for alloys within the commerciallncoloy 800 specification range. The ductility peak at ∼0.1% Ti is not fully understood; it might be best explained by two independent mechanisms, one accounting for the increasing ductility with increasing titanium at levels less than 0.1% and the other explaining the decreasing ductility at higher titanium levels. The grain size effect may be due to differences in the distribution of helium and/or trace elements or to a grain-size dependent variation in stresses necessary for grain boundary fracture. Higher postirradiation ductility at lower stresses could be due to stress relaxation, grain boundary migration, or less pronounced stress-induced growth of helium bubbles present in the grain boundaries.