Patients suffering from recurrent tonsillitis (RT), hyperplastic tonsillitis (HT) or idiopathic tonsillar hyperplasia (ITH) were compared in immunological studies with subjects showing clinically normal palatine tonsils. Serum concentrations of immunoglobulins, particularly IgG, were increased in association with tonsillitis. Conversely, the number of IgG-, IgA- and IgM-producing immunocytes was reduced per tonsillar tissue unit. The density of such cells was decreased in the germinal centres of the lymphoid follicles, in the extrafollicular areas and in the reticular parts of the crypt epithelium. However, only small changes in immunocyte class ratios were observed, excepting a significantly raised proportion of IgD-producing cells. The immunohistochemical results indicated a defect in the tonsillar immunological function associated with disease, perhaps as a result of inadequate stimulatory signals for local maturation of B-cell clones. An influence of low age on this result in the HT and ITH groups could not be excluded. In the RT group there was a significant negative correlation between the tonsillar density of IgA-producing cells and the rate of synthesis of serum IgA. This may indicate that compensatory mechanisms are reflected in inversely related levels of local and systemic IgA synthesis.