We investigated the excitonic or free-carrier nature of lasing in ZnCdSe/ZnSe quantum wells through stimulated emission measurement in high magnetic field and at by pump-probe nonlinear transmission. A free-exciton to free-carrier gas-phase transition with increasing photoinjected density was found to determine the recombination mechanism responsible for lasing. A self-consistent theory based on the mass-action law, including many-body renormalization of the exciton binding energy and of the hot-carrier effects, was used to extract the phase diagram of the exciton -plasma gas from the pump-probe experiments. In deeper walls exciton localization at sample inhomogeneities is demonstrated by time-resolved photoluminescence experiments. Correspondingly, the exciton to free-carrier phase transition does not change the dominant excitonic character of lasing, which reflects the increasing screening threshold of localized excitons. (C) 1996 Optical Society of America