A sensitive sandwich enzyme-linked immunosorbent assay (ELISA) for quantitating yellowtail immunoglobulin (Ig) was developed. In this assay, sera of 0.1 to 1.0μl was enough to quantitate the concentration of Ig. The minimal detectable amount of sera Ig was 0.40±0.17ng/test. Coefficients of variation for within and between assay ranged from 6.8 to 18.8%. When sera Ig from 56 healthy yellowtails were measured, the concentration of Ig for 10 to 110g fish was between 0.10 to 1.84 mg/ml and the concentration for 300 to 600g fish was between 0.96 to 3.04 mg/ml. This result may show a tendency of higher concentration of Ig in sera of larger fish.