Na,K-ATPase purified from rat kidney or bovine choroid plexus was preincubated with the antibodies shown or with control IgG, and then ouabain-sensitive ATP hydrolytic activity was measured. n,Number of independent determinations; ND, not determined. The specific activities for different choroid plexus Na,K-ATPase preparations used averaged 15.2 ± 2.0 μmol · hr⁻¹ · mg of protein⁻¹ (when measured at 5 mmNa⁺) and 37.5 μmol · hr⁻¹ · mg of protein⁻¹ (when measured at 100 mmNa⁺). Because no differences between results at 5 or 100 mm Na⁺ were observed, the data were combined. A paired t test was applied to the results for kidney (γ instead of PLM) compared with choroid plexus. For the antibody PLM-C2, the results were significant at p = 0.021 (p < 0.05).