Water-soluble polysaccharides were extracted from the fruit pulps of Melia azedarach and some columns were undertaken to isolate the major polysaccharide (MPS-III). Its structural features were elucidated by IR analysis, carbohydrate analysis, periodate oxidation, Smith degradation, methylation analysis and NMR spectroscopy. Then it was evaluated for the cytotoxic activity in vitro against four human cell lines, using the sulphorhodamine B assay. The data obtained indicated that MPS-III contains a alpha(1-->4) main chain backbone composed of arabinose, mannose in a molar ratio of 1.31:1.0 and has alpha(1-->6) branch structure. And MPS-III showed a strong cytotoxic activity in the BGC-823 cell line.