The onset of ability to produce haemolytic antibody to sheep erythrocytes does not appear in hamsters until 11 days after birth. However, when mouse erythrocytes were used as antigen an earlier onset of immunocompetence at 6 days after birth was detected. The hamster can make haemolytic antibody only 9.5 days after the appearance of lymphocytes in the thymus—this is sooner than any other species. This capacity is compared with other species on the basis of physiological development by age-equivalence.