Humoral as well as cellular immunological functions were examined before and after tonsillectomy in 84 patients with recurrent tonsillitis. The patients were divided into 3 groups: Group A: under 8, Group B: 9-15, Group C: over 16 years.Elevated IgG was observed in 29.8% of Group A, in whom IgG returned to normal after tonsillectomy. IgG was low in 25% of Group B before tonsillectomy, which remained still low one month after tonsillectomy. Higher incidence of low CH50 was observed in elder patients suggesting that chronic pathological changes exist in tonsils of these age groups. The low CH50 value tended to become normal after tonsillectomy. C3c was reduced before tonsillectomy in 69% of Group A, 60% of Group B and 60% of Group C, respectively. C3c, however, remained unchanged after tonsillectomy. The subpopulation of the T lymphocyte and the rate of lymphocyte blastic transformations estimated by PHA-P or PWM were low before tonsillectomy and became normal after tonsillectomy in many patients of the 3 age groups. While the peripheral T lymphocytes decreased in number in elder patients, the number of the T lymphocytes in removed tonsils increased as the age of the patients advanced.This study would suggest that decision has to be made carefully whether tonsillectomy is indicated in Group A and B depending upon the results of immunological investigations. It is also suggested that immunological laboratory tests such as immunoglobulins or CH50 should be serially carried out after tonsillectomy.