A radioimmunoassay method for the quantitative determination of plasma 2-hydroxyestriol has been developed. For the purpose of obtaining antiserum for this compound, 6-oxo-2-hydroxyestriol was converted to its 6-(O-carboxymethyl)oxime derivative and then coupled with bovine serum albumin by means of the mixed anhydride technique. The immunization of rabbits with this conjugate produced anti-2-hydroxyestriol antiserum which showed high affinity and specificity with low cross-reactivities to structurally related estrogens. After extraction of pregnancy plasma with ethyl acetate and purification on a Sephadex LH-20 column, 2-hydroxyestriol was measured by radioimmunoassay. The following mean concentrations were found in pregnancy plasma : 1st trimester (28.4 pg/ml), 2nd trimester (37.7 pg/ml), and 3rd trimester (104.1 pg/ml).