A fluorescein isothiocyanate (FITC) technique and one based on peroxidase-antiperoxidase (PAP) were used to study the distribution of immunoglobulin (Ig) in cryostat and paraffin sections of human tonsil. Trypsin and other proteolytic enzymes were used to 'unmask' the antigen in paraffin sections. The effects of processing, and particularly of fixation, on the immunohistochemical response of tissues were studied. The FITC and PAP methods detected Ig in paraffin and cryostat sections equally well. The distribution of the antigen was the same with both methods but the PAP method was the more informative. Formaldehyde-sucrose solution proved more suitable for fixing tissues for immunohistochemistry than glutaraldehyde. Trypsin revealed antigen in parraffin sections more efficiently than pepsin, papain, or pronase. Surface Ig (s-Ig) could be demonstrated in trypsinised paraffin sections but less effectively than in cryostat sections. Trypsinised paraffin sections were, however, more suitable for intracellular Ig (c-Ig) than cryostat sections although the performance of cryostat sections could be improved by prior fixation with a coagulative fixative.