Intracellular immunoglobulin has been demonstrated in human palatine tonsils by the unlabelled antibody peroxidase-antiperoxidase complex (PAP) method in which rabbit antiserum to a range of human immunoglobulins (Igs) was linked to the PAP complex by an intermediate stage of swine antiserum to rabbit Ig. The effects of different methods of fixation and processing have been compared, formol-saline fixation giving the best results. The PAP technique proved greatly superior to the fluorescein isothiocyanate (FITC)-based technique, not only in sensitivity but in permitting study of the finer histological and cytological features. The lymphoid follicles are shown to have three distinct zones, two forming the follicle centre (zones (a) and (b)), and the third (zone (c)) the lymphocyte cap. Ig synthesis appeared to begin in the cells in zone (b). IgG, IgA, IgM, IgE and IgD were present in all tonsils, with IgG predominating, confirming that the tonsil resembles lymph nodes more closely than it does alimentary lymphoid tissue. Some follicles contained more than one type of Ig. The tonsil appears to have a well-developed T-dependent area, the lymphoid follicles forming a B-cell area. The structure of the tonsil would seem to facilitate contact between its lymphoid tissue and antigens in the crypts, and it is postulated that some T cells within the crypt epithelium, after contact with antigen, may leave the tonsil by the efferent lymphatics and enter the peripheral circulation by the thoracic duct, whilst other primed T cells interact with B cells in the follicle centres. Some B cells may then start to synthesize immunoglobulin, whilst others become memory cells in the lymphocyte 'cap' of the follicle.