Nitrosation and acetylation, two histochemical blocking procedures for amino groups, were used to establish the extent to which these groups intervene in the antigen-antibody reaction in immunohistochemistry. We used the peroxidase-antiperoxidase method (PAP) to demonstrate lysozyme in Paneth cells and in lamina propria mononucleocytes of human small intestine as a model system. We studied the relationship of these groups to fixation, concentration of the primary antiserum, and length of blockade, as well as the possibility of reversing blockade as proof of specificity. Our findings support the contention that amino groups are also an important factor in antigen-antibody binding, even in fixed tissue. Fixatives influence the binding process in many ways, with acetylation producing a more successful result than nitrosation in tissue fixed in Bouin without acetic acid, whereas the reverse is true in formaldehyde-fixed tissue.