Mass concrete at Friant Dam was produced with high- and low-alkali cements both with and without natural pozzolan (pumicite). Recent and previous investigations indicate (a) concrete deterioration due to alkali-aggregate reaction has taken place to varying extent and degree within the dam; (b) compressive strength and modulus of elasticity have decreased due to alkali-aggregate reaction; (c) preliminary tests suggest alkalies are not present in the concrete in sufficient quantity to contribute to expansive reactions; and (d) the potentially deleteriously alkali-reactive aggregate is of andesite affinity.