A polyclonal antiserum, produced in sheep and reactive against purified human leucocyte elastase, has been applied to paraffin sections from a range of human tissues by means of the indirect immunoperoxidase method. Striking and consistent results have been obtained. Normal or inflammatory granulocytes were intensely positive, the reaction being "blocked" by pure elastase. Activity was not seen in other sites, including histiocytic reticulum and lymphoid cells, although some weaker reaction was present in gastric and ileal lining epithelium. Strong reactivity was also seen in the cells of acute and chronic myeloid leukaemia and in extramedullary haemopoiesis. The advantages of this technique are compared with those for muramidase (lysozyme), alpha 1-antitrypsin, and naphthol-AS-D-chloroacetate esterase.