Treatment of transformed breast duct epithelial cells with IL-6 produces a unique cellular phenotype characterized by diminished proliferation and increased motility. Human ductal carcinoma cells (T-47D and ZR-75-1 lines) are typically epithelioid in shape and form compact colonies in culture. Time-lapse cinemicrography shows that some untreated cells can transiently become fusiform or stellate in shape and separate from each other within a colony, but they usually rejoin their neighbors. While IL-6 suppresses the proliferation of these carcinoma cells, the IL-6-treated cells generally become stellate or fusiform and show increased motility. These changes persist as long as the cells are exposed to IL-6. This results in the dispersal of cells within colonies. The effects on cell growth, shape, and motility are reversible upon removal of IL-6. IL-6-treated T-47D cells display diminished adherens-type cell junctions, as indicated by markedly decreased vinculin-containing adhesions and intercellular desmosomal attachments. The effects on ZR-75-1 cell shape, colony number, and DNA synthesis are dependent on IL-6 concentration in the range from 0.15 to 15 ng/ml. Higher concentrations are required in T-47D cells for equivalent effects. Anti-IL-6 immune serum blocks IL-6 action. IL-6 represents a well-characterized molecule that regulates both the proliferation and junction-forming ability of breast ductal carcinoma cells.