Islet transplantation for type 1 diabetes can enable the achievement of near-normal glycemic control without severe hypoglycemic episodes. How much an islet (β-cell) graft may be contributing to glycemic control can be quantified by stimulatory tests of insulin (or C-peptide) secretion. Glucose-potentiation of arginine-induced insulin secretion provides a measure of functional β-cell mass, the β-cell secretory capacity, as either AIR pot or AIR max, but requires conduct of a hyperglycemic clamp. We sought to determine whether acute insulin responses to intravenous glucose (AIR glu) or arginine (AIR arg) could predict β-cell secretory capacity in islet recipients. AIR arg was a better predictor of both AIR pot and AIR max (n= 10, r 2= 0.98, P< 0.0001 and n= 7, r 2= 0.97, P< 0.0001) than was AIR glu (n= 9, r 2= 0.78, P= 0.002 and n= 6, r 2= 0.76, P= 0.02). Also, the measures of β-cell secretory capacity were highly correlated (n= 7, r 2= 0.98, P< 0.0001). These results support the use of AIR arg as a surrogate indicator of β-cell secretory capacity in islet transplantation.