An indirect enzyme-linked immunosorbent assay (ELISA) was developed to measure the concentration of abscisic acid (ABA) in soybean hypocotyl tissues after inoculation with Phytophthora megasperma f. sp. glycinea. The assay, which uses monoclonal antibodies, has a measuring range of 5-250 pg of ABA. Validation of the assay with high-performance liquid chromatography showed that the assay is a sensitive and reliable method for the determination of ABA in crude tissue extracts. In an incompatible interaction the concentration of ABA in lesions was reduced to 32% of uninoculated control levels by 6 hr after inoculation.