Standardized ileal digestibility (SID) of AA in protein ingredients and grains was determined in weaned piglets (5 kg initial BW) using the difference method. Animals were fitted with a simple ileal T-cannula on day 24 or 25 of age. A synthetic diet based on corn (Zea mays) starch and casein was either supplemented with an extruded soybean (Glycine max) meal (ESM), rice (Oryza sativa) protein concentrate (RPC), full fat heat-treated soybeans (SB), corn, barley (Hordeum vulgare), or wheat (Triticum aestivum). The contribution of CP and AA from casein and from the assay feed ingredients to the assay diet averaged 50% each. Each diet was fed to 6 animals according to a row-column design with 3 periods. Apparent ileal digestibility was corrected for basal ileal endogenous losses of AA to obtain SID values. Between RPC and soy products, differences (P < 0.05) in SID of most AA such as Lys, Met, and Thr were greater than within soybean products (ESM and SB). The SID of indispensable AA did not differ (P > 0.05) between barley and wheat, except for Met (P < 0.05). Furthermore, SID of Arg, His, Ile, Phe, and Trp was lower (P < 0.05) in corn than wheat. In conclusion, SID of most AA in the present study with piglets was lower than tabulated SID values determined in studies with grower-finisher pigs. Furthermore, the use of RPC, despite its high CP content, in diets for weaned piglets is limited due to its low SID of AA.