A comparative immunohistochemical study of intermediate filament expression in normal parotid glands and pleomorphic adenomas (PA) was performed using material fixed in a modified methacarn fixative. The normal myoepithelial cells of acini stained only with monoclonal antibodies 312C8-1 (cytokeratin (CK) 14) and 4.62 (CK 19) while myoepithelial/basal cells of ducts also reacted with antibodies 8.12 (CK 13, 16), 8.60 (CK 10, 11, +/- 1), and PKK1 (CK 7, 8, 17, 18). Normal duct luminal cells showed a different CK profile, reacting consistently with ECK, a polyclonal antibody to epidermal prekeratin (CK 3,6), and monoclonal antibodies 4.62, PKK1 and 8.60. In PA, tumour cells at the periphery of ducts, in solid areas, and at the edge of myxoid regions all had CK profiles similar to normal myoepithelial/basal cells except that antibody 4.62 was generally negative. Vimentin and glial fibrillary acidic protein (GFAP) were uniformly negative in normal parotids but showed variable (often strong) reactivity with some cells in chondroid, myxoid and solid areas of PA. A surprising feature of most PA was the variability of CK subtype expression not only from one case to another but also within morphologically similar areas of the same specimen. These results suggest that the morphology of PA is the result of diversity of tumour cell differentiation rather than the processes implicit in a reserve cell histogenetic model.