An enzyme immunoassay for the determination of potassium oxonate in the plasma has been developed. The procedure is based on a competitive enzyme-linked immunosorbent assay using the second antibody solid phase method. The antiserum for potassium oxonate was prepared using oxonic acid 6-carboxypentylamide-BSA conjugate as immunogen. The specific antibody for oxonic acid was isolated from the antiserum using oxonic acid 6-carboxypentylamide immobilized immunosorbent gel. The purified antibody resulted in high sensitivity and low cross-reactivity as compared with the unpurified antiserum. Potassium oxonate in the plasma could be assayed in the range from 20 to 1000 ng/mI by the proposed EIA. The recovery was ranged from 82 to 117% and the coefficient of variation was from 6.6 to 14.7% (n=6).