The processes involved in the stimulated emission by photopumping in (Zn,Cd)Se-ZnSe multiple quantum wells have been investigated at 77 K for a series of different well widths. It has been shown by means of photoluminescence-excitation spectroscopy that the confined excitons in the well play an important role in determining the lasing mechanism. The optical gain just above the lasing threshold is attributed to the recombination of an exciton accompanied by emission of one LO phonon. Far above threshold, inelastic exciton-exciton scattering processes contribute significantly to the gain. © 1993 The American Physical Society.