Perinuclear buttons were identified in cultured epithelial cell lines, with monoclonal antibodies to vimentin, following treatment with drugs which disrupt intermediate filaments. These buttons appeared after 10 min when cells were exposed to 10 micrograms/ml vinblastine or cyclohexamide. After the drug was removed vimentin-containing filaments radiated from this button through the cytoplasm. In some cells peripheral buttons appeared, up to six in one cell. Within 1 h the filaments had reformed to a normal pattern for intermediate filaments. These buttons did not form in mesenchymal cell lines, mouse; rat and 3T3 fibroblasts. They appeared only in epithelial cell lines. Monoclonal antibodies to keratin and tubulin did not stain these buttons. These appear to be discrete entities separate from the microtubule organizing centre and to be found only in cultured epithelial cell lines. This structure may act as the organization centre for vimentin in cultured epithelial cell lines.