In sectional sign 1 of this paper, we characterize the isomorphism pro
perty of nonstandard universes in terms of the realization of some sec
ond-order types in model theory. In sectional sign 2, several applicat
ions are given. One of the applications answers a question of D. Ross
in [this JouRNAL, vol. 55 (1990), pp. 1233-1242) about infinite Loeb m
easure spaces.