The antigen binding capacity (ABC) of mouse and rabbit antisera to horseradish peroxidase (HRP) was measured by the Farr technique, originally introduced for bovine serum albumin. Results obtained by this method are independent of other antibody interactions (precipitation, formation of immune complexes, effect on enzyme activity). Commercially available HRP has variable solubility in ammonium sulfate solutions; almost 40% was precipitated at 50% saturation. Accordingly, the fraction of HRP soluble at 60% saturation was used, and this was labelled with 125I2 using the HRP oxidizing activity. Under these test conditions, accurate and reproducible results were obtained with protein concentrations (as N/ml) of the order to 0.07 mug (antibody) and 0.01 mug (antigen) respectively. This high sensitivity is superior to that achieved by passive hemagglutination, especially when titrating antibody levels in mouse sera.