Our continuing interest on Garcinia and Mesua species has led us to carry out a detail study on the chemistry of the root bark of Garcinia mangostana (Guttiferae) since this part of the plant has not been investigated before, and the strm bark of Mesua corneri (Guttiferae) an uninvestigated species. This study has yielded six xanthones, ${alpha}-mangostin$ (1), ${eta}-mangostin$ (2), ${gamma}-mangostin$ (3), garcinone-D (4), mangostanol (5) and gartanin (6) from Garcinia mangostana and two xanthones rubraxanthone (7) and inophyllin B (8) from Mesua corneri. Structural elucidations were achieved using $^1H,;^{13}C$ NMR and MS data. The crude hexane and chloroform extracts of the root bark of Garcinia mangostana and the hexane extract of the stem bark of Mesua corneri were found to be active against CEM-SS cell lines with $IC_{50}$ values less than $30;{mu}g/ml$. Moreover, ${gamma}-mangostin$ gave a very low $LC_{50}$ value of $4.7;{mu}g/ml$ while rubraxanthone gave an $LC_{50}$ value of $5.0;{mu}g/ml$ indicating these two compounds to be potential lead compounds for anti-cancer activity against the CEM-SS cell line. This paper reports the isolation and identification of these compounds as well as bioassay data for the crude extracts, ${gamma}-mangostin$ and rubraxanthone.