GST-π was purified from human placenta and its antisurum was raised in rabbits. The antibody IgG was purified and degraded into Fab′ fragment which was conjugated with horseradish peroxidase (HRP) using N-succinimidyl-4-(N-maleimido-methyl) cyclohexane-carboxylate (SMCC) as crosslinking reagent to produce Fab′-HRP conjugate. A sandwich ELISA was established for the microquantitative determination of GST-π. The sentitivity, was 11pg/ tube, which was far more sensitive than the radioimmunoassay so far reported. Using this method, the serum GST-π of 41 cases of normal adult was found to be 1.06+0.94ng/ml. The upper limit of the normal value was 2.5ng/ml. In 30 cases of primary hepatocarcinoma, the level of serum GST-π was 24.4+17.4ng/ml, which was 23 times higher than the normal average value (p＜0.01). The positive rate was 90%. In contrast serum GST-π in 25 cases of chronic hepatitis was determined to be 1.74 + 1.16ng/ml, which was not significantly different from the normal value (p＞0.05). The pseudo-positive rate was 12.0%.