An immunoperoxidase procedure is described for staining intracellular leucocyte antigens in peripheral blood and bone marrow smears. Brief exposure of cell smears to a buffered formol acetone mixture was found to give optimal fixation, combining good cellular morphology with preservation of antigenic reactivity. The immunoperoxidase method is superior to immunofluorescence in that it provides a permanent preparation which can be counterstained with orthodox reagents and viewed by conventional light microscopy. In addition the technique is considerably more sensitive than immunofluorescence procedures. Immunoglobulin was demonstrated in plasma cells, Türk cells and a minority of peripheral blood lymphocytes. Lysozyme was found in cells of the neutrophil series from promyelocytes to mature granulocytes. Monocytes stained for lysozyme but the reaction was less intense than in neutrophils and some monocytes were devoid of activity. Lactoferrin stained strongly in mature neutrophil polymorphs and metamyelocytes, but was weak or absent in earlier myeloid cells. These reaction patterns are in keeping with previous reports on the distribution of these antigens in human leucocytes. In the case of immunoglobulin and lysozyme it was possible to abolish leucocyte staining by incubation of the specific antisera with the appropriate purified antigen, providing additional proof of the specificity of the reactions. Anti-ferritin antisera stained granulocytes and myeloid precursors strongly, and reached weakly with a minority of monocytes. These latter observations are not entirely in accordance with published data on the leucocyte distribution of ferritin and may be attributed to antibody activity of unknown specificity in the anti-ferritin antiserum.