An investigation of the electric birefringence of aqueous suspensions of purple membrane fragments was carried out. We have determined values for the permanent dipole moment (p approximately 10(-24) C m) and for the membrane polarizability (DELTAgamma approximately 10(-28) F m2) of the purple membrane fragment. These values are in agreement with previous measurements made by other authors using different electro-optical techniques. In addition, we have determined that the optical anisotropy Ag of the purple membranes is almost-equal-to 10(-3). We have probed the nature of the electric field orientation function for the purple membrane fragments, and we suggest a method for determination of p and DELTAgamma based on an experimentally measured value of the orienting dc clectric field strength at which the electro-optic effect goes to zero.