Sodium butyrate is a potent inducer of leukemic cell differentiation. This agent also affects epithelial cell proliferation and gene expression. The present study demonstrates that sodium butyrate slows proliferation and induces morphologic changes in human MCF-7 breast carcinoma cells. The results also demonstrate that butyrate-treated MCF-7 cells have enhanced expression of carcinoembryonic antigen. These effects are related to butyrate concentration and duration of exposure. The present findings may provide a model for studying maturation of human breast cancer cells.