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Tissue slices (500 to 1000 microns thick) of archival formalin-fixed, paraffin-embedded breast tissue were immunostained by a cytokeratin antibody (MNF116) using a streptavidin-biotin complex procedure. The technique requires prolonged exposure of tissue slices to the reagents. Use of the detergent Triton X-100 facilitated penetration of high molecular weight reagents through the tissue slices. Fifty of 58 slices 500 microns thick (86%) showed good to excellent immunostaining, and 13 of 20 slices 1000 microns thick (65%) showed similar staining. Omission of the primary antibody eliminated any immunostaining. Comparison with corresponding Haematoxylin staining of the thick slices (the conventional procedure for such breast tissue slices) showed that thick-slice cytokeratin immunostaining markedly improved visualization of the epithelial structure in normal lobules and invasive carcinomas. Although the immunohistochemical technique takes 33 days for completion, the quality of the epithelial images outweighs this disadvantage.