- We demonstrate the application of electromagnetic-field-induced transparency and quantum interference effects in the cooperative phenomenon, such as optical bistability. The control field used in tandem with the usual electromagnetic field of the two-level scheme results in a considerable lowering of the threshold intensity. We discuss the transient response of the system in the mean-field limit and describe the regression to the steady state when perturbed away from it; the regression exponent is itself dependent on the control field. We also demonstrate the possibility of control-field-induced multistability in two-level systems. 1996 The American Physical Society.