BACKGROUND Breast cancer is the second leading cause of cancer deaths in US women. We evaluated two novel compounds, piperidinyl-diethylstilbestrol (DES) and pyrrolidinyl-diethylstilbestrol (DES) for cytotoxicity against brine shrimp larvae, MCF-7 and rat normal liver cells. METHODS In vivo cytotoxicity was evaluated against shrimp larvae for 24 h, while in vitro cell toxicity was evaluated by dye binding crystal-violet method after 48 h. The role of these compounds on different phases of the cell cycle was assessed by flow cytometry. RESULTS In shrimp assay, piperidinyl-DES and pyrrolidinyl-DES were potent with 50% effective dose (ED(50)) values of 7.9+/-0.38 and 15.6+/-1.3 microM, respectively. In MCF-7 and normal liver cells, the 50% lethal concentration (LC(50)) values were 19.7+/-0.95, 17.6+/-0.4 microM and 35.1 and >50 microM, respectively. Cell cycle analyses indicated that MCF-7 cells were arrested at the G(0)/G(1) stage with these compounds. CONCLUSIONS The results indicate that pyrrolidinyl-DES possesses highly selective, potent anticancer activity.