In order to develop useful optical probes of a high-mobility 2D electron gas, we have investigated the many-body response of a photoinjected hole with the electron system. In asymmetric GaAs/(Ga, Al)As single quantum wells strong many-body exciton effects, Fermi-edge singularities, are observed, especially under conditions where the electron Fermi level is near the nc = 2 conduction subband. The subband proximity effectively transforms the edge singularity to a many-body exciton at k = 0, where it is strongly coupled to optical fields (photoluminescence). In applied magnetic fields very large 1 B periodic amplitude oscillations are seen in the edge singularity, reflecting the changes in the stability of the magnetoexciton depending on the position of the electron Fermi level within the Landau manifold. © 1992.