Summary. In the present paper it is demonstrated that the pH at which rocket immunoelectrophoresis can be performed is not restricted to the conventional value of 8.6. By carbamylation the pI of rabbit antibodies was lowered until their mean electrophoretic migration in an agarose gel at pH 5.0 was zero. Using such carbamylated antibodies, IgG from normal human sera were quantitated by rocket immunoelectrophoresis at pH 5.0. The results obtained were shown to be concordant with the IgG concentrations determined at pH 8.6 using carbamylated antigen as described by Weeke (134). The advantage of utilizing carbamylated antibodies for quantitation of IgG in routine laboratory work is pointed out, and additional applications are stated.