We have observed second harmonic generation (SHG) at 532 nm in randomly oriented suspensions:of the purple membrane of Halobacterium halobium. The intensity of the second harmonic can be switched by photoexcitation of the bacteriorhodopsin (BR) chromophore at 570 nm. The SHG intensity decreases in <1 mu s and reaches a minimum in <1 ms with the formation of the M state of BR. For dilute samples, we have observed photoinduced changes in the SHG intensity of up to 90%. At low concentrations, the second harmonic intensity in light-adapted BR depends linearly on concentration, while showing saturation at higher concentrations. The linear concentration dependence Shows that coherence of the second-order polarization between membrane particles is weak. At the low concentrations used, the SHG time dependence cannot be attributed to absorption of the 532-nm second harmonic. The fact that the SHG signal is almost entirely abolished with the formation of the M state suggests that the BR signal is strongly resonance enhanced by a two-photon resonance at 532 nm.