酶免疫测定（enzyme immunoassay, EIA）或免疫酶技术（immunoenzymatic technique）是指用酶标记抗体或酶标记抗体进行的抗原抗体反应。它采用抗原与抗体的特异反应与酶连接，然后通过酶与底物产生颜色反应，用于定量测定。

目前常用的方法称为酶联免疫吸附法（enzyme-linked immunosorbent assay, ELISA），其方法简单，方便讯速，特异性强。

ELISA是由抗原（抗体）先结合在固相载体上，但仍保留其免疫活性，然后加一种抗体（抗原）与酶结合成的偶联物（标记物），此偶联物仍保留其原免疫活性与酶活性，当偶联物与固相载体上的抗原（抗体）反应后，再加上酶的相应底物，即起催化水解或氧化还原反应而呈颜色。其所生成的颜色深浅与欲测的抗原（抗体）含量成正比。

Fig 8-8 ELISA. 1. Antigen in saline is incubated on a plastic plate or tube, and small quantities become absorbed onto the plastic surface. 2. Free antigen is washed away. (The plate may then be blocked with excess of an irrelevant protein to prevent any subsequent non-specific binding of proteins). 3. Test antibody is added, which binds to the antigen. 4. Unbound proteins are washed away. 5. The antibody is detected by a ligand. The ligand is a molecule which can detect the antibody and is covalently coupled to an enzyme such as peroxidase. 6. This binds the test antibody and after free ligand is washed away. 7. The bound ligand is visualized by the addition of chromogen-a colourless substrate which is acted on by the enzyme portion of the ligand to produce a coloured end-product. 8. A developed plate is shown in the lower panel. The amount of test antibody is measured by assessing the amount of coloured end-product by optical density scanning of the plate.