We have developed and evaluated a new staining method for use in electrophoretic analysis of alpha-amylase (AMY, EC3. 2. 1. 1) isoenzymes in serum. Our method based on the colorimetric determination of formazane dye formed by enzyme-coupled reactions and maltopentaose was used as a substrate. We have confirmed that this method was sensitive and stable. Liquid reagents are used, so the automatic analysis of AMY isoenzymes using a fully-automated electrophoresis system will become possible.