White mice were immunized with extracts of ovaries or tests of Culex pipiens either infected or not infected with Wolbachia pipientis. Immunoelectrophoresis of hyperimmune sera to these extracts revealed 2 antigenic components in infected gonads and 1 in non-infected gonads. The findings suggest that 1 component is related to ovarian or testicular tissue and the other to the symbionts. Absorption of the antibody to infected ovaries by uninfected ovarian tissue removed all activity of the serum.