AbstractThis study aimed to investigate the chemical constituents from Alphonsea cylindrica as well as their cytotoxic and xanthine oxidase inhibitory activities. The chemical compounds were isolated and purified by various chromatographic techniques and their structures were elucidated via modern spectroscopic techniques including NMR, MS, IR, UV and comparison with literature. A study on the barks of A. cylindrica has found eleven known compounds were identified as stigmasterol (1), isoursuline (2), cyathocaline (3), kinabaline (4), muniranine (5), isooncodine (6), iraqiine (7), O-methylmoschatoline (8), kareemine (9), atherospermidine (10) and N-methylouregidione (11). The cytotoxic effect of hexane, DCM and methanol of bark of A. cylindrica as well as the isolated compounds; 1, 2, 3, 4, 5, 7, 8, 9 and 10 were estimated on MCF-7 human breast cancer cells and isoursuline possessed the most potent inhibitory activity with IC50 value of 33 µg/ml. Eight compounds 1, 3, 4, 5, 6, 8, 10 and 11 di.....