AbstractMicro reactors and micro total analysis systems (µTAS) are recognized as powerful tools for genomics, proteonomics, clinical diagnostics, and environmental testing. In this paper, we describe an enzyme‐linked immunosorbent assay (ELISA) using a new micro reactor with a vertical fluid flow operation. This micro reactor is composed of two reaction vessels stacked along the vertical through PMMA fluid filters (Ø 3 mm). The fluid filters, constructed by deep X‐ray lithography, have 2100 pores (Ø 40μm), and possess valve functions, making it possible to maintain the liquid layers in each reaction vessel. In addition, the liquid can be selectively transferred by air pressure from the upper vessel to the lower, and vice versa. As a model of ELISA using the micro reactor, we undertook to detect mouse immunoglobulin (IgG). We bound goat anti‐IgG antibody to the surface of the PMMA filters and assayed the IgG by ELISA using an anti‐IgG antibody/peroxidase conjugate. We found that the mouse IgG (100 ng/ml) was quantitatively detected within 45 minutes of the analytical period, or about one‐third of the period required for the conventional method using micro titer plates. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(4): 50–57, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10137