This novel assay uses a high affinity monoclonal capture antibody to selectively isolate carboxylesterase 1 (CES1) from liver tissue or cell samples. After isolation and subsequent measurement of the enzyme's functional activity, the quantity of isolated CES1 is measured in the same well by adding a second monoclonal detector antibody, which is quantified using a colorimetric label (HRP). Both reactions take place in time-dependent manners proportional to the amount of enzyme captured in each well. By combining activity and quantity measurements, the enzyme's relative specific activity can be determined. Specific activity is useful for measuring up or down regulation of activity by site-specific modification or damage, and in response to specific inhibitors.