The interaction of antibodies with a distinct antigenic determinant in the region (99-126) of staphylococcal nuclease has been found to produce a soluble, inactive antibody-nuclease complex. The reaction has been followed spectrophotometrically, using the rate of hydrolysis of substrate DNA as a measure of residual, free nuclease concentration. This analysis has provided the following kinetic and equilibrium constants for the antibody-antigen interaction: Kass = 8.3 X 108 M-1, Kon =4.1 X 105 m-1 sec-1, A-offf t = 4.9 X 10-4 sec-1. Conversely, measurement of changes in nuclease activity can be used as a rapid and sensitive assay of antibody concentration. The Scatchard plot of the equilibrium inactivation data bends sharply near r = 1, which may reflect heterogeneity of the antibodies or may indicate possible steric interference in the binding of a second nuclease molecule to the bivalent antibody molecule.