We propose a simplified version of a cooperative lattice membrane model given by Grodsky to explain observed Ca++ efflux windowing effects from cell membranes exposed to electromagenetic radiation. Assuming that field induced conformational interactions occur only between bistable receptor sites and glycoprotein Ca++ sites on the surface of the membrane, the model is shown to be equivalent to an Ising model. This model is known to have a phase transition to an ordered state in which a macroscopic number of Ca++ sites are either occupied or unoccupied. We identify such states with enhanced Ca++ efflux from cell membranes. By further assuming an averaged signal, sinusoidally varying coupling between receptor and Ca++ sites and a power-law dependence of the characteristic time constant on the induced power-density of the applied field, we show that the model is consistent with published experimental results on power density windowing effects for particular values of model parameters. For these parameter values, the model predicts further power densities where windowing effects may be observed under appropriate conditions. © 2000 Wiley-Liss, Inc.