Patients suffering from recurrent tonsillitis (RT), hyperplastic tonsillitis (HT), or idopathic tonsillar hyperplasia (ITH) were compared in immunological studies with subjects showing clinically normal tonsils. Serum concentration of immunoglobulins, particularly IgG, were found to be increased in association with tonsillitis. Conversely, the number of IgG-, IgA- and IgM-producing immunocytes per tissue unit was reduced in the germinal centres of lymphoid follicles, in the extrafollicular areas, and in the reticular parts of the crypt epithelium. The overall percentage distribution of these cells was normally 65:30:3.5:1.2 for the IgG, IgA, IgM and IgD classes, respectively. In RT these figures were 53:39:4.7:4.4; in HT, 67:25:4.0:4.5; and in IHT, 50 : 33 : 7.2 : 10. Thus, there were only small alterations in the immunocyte class proportions, except for a significant relative increase in IgD-producing cells. The results indicate that there is a functional defect of the tonsils in association with disease. The relative accumulation of IgD cells is probably explained by an inadequate local maturational process in B-cell system, altough some influence of low age cannot be excluded in the HT and ITH groups.