The amplification and overexpression of the HER-2/neu proto-oncogene, which encodes the tyrosine kinase receptor p185neu, have been observed frequently in tumors from human breast cancer patients and are correlated with poor prognosis. To explore the potential of chemotherapy directed at the tyrosine kinase of p185neu, we have found that emodin (3-methyl-1,6,8-trihydroxyanthraquinone), a tyrosine kinase inhibitor, suppresses autophosphorylation and transphosphorylation activities of HER-2/neu tyrosine kinase, resulting in tyrosine hypophosphorylation of p185neu in HER-2/neu-overexpressing breast cancer cells. Emodin, at a 40-microM concentration, which repressed tyrosine kinase of p185neu, efficiently inhibited both anchorage-dependent and anchorage-independent growth of HER-2/neu-overexpressing breast cancer cells. However, the inhibition was much less effective for those cells expressing basal levels of p185neu under the same conditions. Emodin also induced differentiation of HER-2/neu-overexpressing breast cancer cells by exhibiting a morphological maturation property of large lacy nuclei surrounded by sizable flat cytoplasm and by showing a measurable production of large lipid droplets, which is a marker of mature breast cells. Therefore, our results indicate that emodin inhibits HER-2/neu tyrosine kinase activity and preferentially suppresses growth and induces differentiation of HER-2/neu-overexpressing cancer cells. These results may have chemotherapeutic implications for using emodin to target HER-2/neu-overexpressing cancer cells.