Majority of the currently available anticancer drugs are designed to have selective toxicity to rapidly dividing cells. Among these agents the focus of many studies are compounds obtained from natural products with high therapeutic index. In this study the cytotoxicity of HESA-A, a marine compound, on cancer and normal cells was evaluated. HESA-A was prepared in normal saline as a stock solution (0.8 mg/ml, pH=7.4), sterilized and further diluted to final concentrations of 0.4, 0.2, 0.1 and 0.05 mg/ml. Cells (MDA-MB-468, Hep-2, Hela as cancer cells; L929 and McCoy as normal cells) were grown in completed RPMI 1640 and seeded in 96 well micro plates at a concentration of 1-5 × 104 cells/ml. After incubation for 24 h, different concentrations of HESA-A were added and cells were further incubated for 72 h. Using MTT assay, percent cell survival was determined by ELISA at 540 rim. Doxorubicin was used as a positive control (20 µg/ml). HESA-A (0.4 mg/ml) reduced the number of viable MDA-MB-468 and Hela cells to less than 50%. For Hep-2 cells the IC50 was 0.8 mg/ml. In normal cells IC50 could not be obtained at any given concentrations. These results suggest that HESA-A in therapeutic doses and in a concentration dependent manner inhibits the growth of cancer cells more selectively than normal cells.