Bivalent (IgG) rabbit anti-mouse zona pellucida antibody completely inhibited fertilization in the mouse. Univalent (Fab) zona antibody did not inhibit the fertilizability of eggs. However, the treatment or injection with anti-rabbit IgG serum inhibited the fertilization of eggs pretreated with antiserum Fab or eggs from females injected with antiserum Fab. It is concluded that the inhibition of fertilization by bivalent louse zona antibody depends on the possibility that the antisera are directed against the zona as a whole and not specifically against the receptors for the sperm.