1. Based on a specific binding of labelled inhibitor to the enzyme active centre, a new principle of enzyme assay, inhibitor binding assay (IBA), was developed and applied to measurement of rat serum angiotensin converting enzyme (ACE).

2. Serum diluted 1:50 was incubated with 125I-labelled ACE inhibitor, 351A, at pH 7.0, 37°C, for 2 h. Inhibitor bound to ACE was separated with coated charcoal and results were calculated from a standard curve.

3. The advantages offered by the novel inhibitor binding assay include simplicity, specificity, absence of interference by other enzymes or immunological cross-reactions, and great sensitivity enabling measurement of ACE in concentrations less than 0.1 units/ml.

4. This principle of enzyme assay will not only have potential new applications for research involving ACE but may also be extended to other enzymes.

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