Stem rust is an important disease of wheat and barley. Barley is genetically vulnerable to stem rust with few identified resistance genes and sources. Stem rust resistance breeding is largely based on the gene Rpg1, first incorporated into North American barley cultivars in the 1940s. To identify potentially new resistance sources, the USDA-ARS National Small Grains Barley iCore Collection (1,860 accessions) was evaluated for reaction to stem rust at the seedling and adult stages. The adult stage evaluations were conducted at Njoro, Kenya (race TTKSK/TTKST), and St. Paul, Minnesota (race QCCJB), for two seasons, and the seedling tests (race TTKSK) in the BSL-3 greenhouse at St. Paul, Minnesota. At St. Paul, between 7 and 10% (132-203) of the accessions exhibited resistance, whereas in Kenya, 11-14% (218-261) were resistant. Correlation between years was higher in Kenya (0.60) than it was at St. Paul (0.48). Approximately 15% (277) of the collection gave moderately low to low reactions to TTKSK at the seedling stage. From these initial tests, 290 accessions were chosen based on diversity of reaction, origin of plant material, or stability across environments. These accessions were then further evaluated with a suite of races at the seedling stage to postulate resistance genes. Of these selections, 244 gave reactions suggesting they carry adult plant resistance. The remaining 46 accessions gave low to very low reactions to one or more races. Based on country of origin and resistance spectrum 15 accessions were predicted to have the rpg4/Rpg5 complex, including a subset from Switzerland. The remaining 31 have a reaction spectrum, country of origin or pedigree that does not suggest the presence of the rpg4/Rpg5 complex. Molecular tests will be used to confirm the presence of this complex in these materials.