The effect of reduction by 2-mercaptoethanol (2-ME) on antibody activity was studied in antisera with high and low IgG concentrations. Sera obtained from B10.LP nu/nu mice during a primary response against a rat PVG/c skin graft contained only 2-ME sensitive antibodies. However, when analyzed on sucrose gradients, IgG as well as IgM antibody activity was present. After raising the low serum IgG concentration (0.3-0.7 mg/ml) of these sera by addition of normal mouse serum (5.1 mg IgG/ml), 2-ME resistant antibodies became detectable. Hyperimmune C57BL6 anti PVG/c lymphocyte serum with a high IgG concentration (20.2 mg/ml) and antibody activity predominantly located in the IgG class was not affected by 2-ME treatment. These data show that IgG antibodies, although less susceptible to reduction than IgM antibodies, are not resistant to this treatment. At high IgG concentrations the proportion of inactivated IgG with specific antibody activity will be negligible but at low IgG concentrations the use of this method leads to serious underestimation of IgG antibody activity.