Abstract: Water temperature plays an important role in the antibody production in grass carp. At 15℃, the titer of neutralizing antibody developed very slowly and was less than 1:8 when the grass carp had been reared for 9 week; In contrast, at 20℃ the titer of antibody was 1: 256 three weeks after infection and reached the highest titer of 1:5 270 after 6 weeks; The fish reared at 25℃ showed a level of antibody of 1:570 in one week and the highest titer was more than 1:20, 000. The antibody in grass carp serum is a kind of macroglobulin. it breaks down easilv into smaller polypeptides, which have the same antigenicity as that for the antibody and are similar to human IgG in molecular weight. The results indicated that the antibody of grass carp possesses some characteristics of human IgM.