An electric birefringence study was carried out on aqueous suspensions of the purple membrane of Halobacterium halobium. In addition to the characterization of both native and modified membrane samples, the dependence of electric birefringence on p H and ionic strength was also investigated. The results indicate that purple membrane shows electric birefringence a t a field strength a s low as 200 V/cm. T h e permanent dipole moment and polarizability ranged from 20,500 debyes and 1.01 X cm3 for a purple membrane concentration of 0.40 mg/mI, to 41,000 debyes and 2.05 X cm:' for a concentration of 0.80 mg/mL. It was also found that removal of the retinyl group of bacteriorhodopsin substantially decreases but does not eliminate the electric birefringence of the membrane. The solubili- zation of the membrane by Triton X-100, however, completely abolishes the electric birefringence. These experiments indicate that there is an interaction between adjacent bacteriorhodopsin molecules within the purple membrane via the retinyl chromophore moiety that builds u p the permanent dipole moment. They also suggest that there are two types of response when purple membrane suspensions are placed in an electric field. One is an alignment of the disk-shaped particles with the field. Theother is a stacking of the particles following their alignment by the electric field, which is promoted by the induced dipole mo- ment.