Preparations from Uncaria tomentosa, a South American Rubiaceae, have been used in the Peruvian traditional medicine for the treatment of infective, inflammatory and tumoral processes. In this study, the pentacyclic oxindole alkaloid mitraphylline was isolated from the dried inner bark of this plant species, and its structure elucidated by analysis of NMR spectroscopic data. Mitraphylline was differentially identified from its stereoisomeric pair isomitraphylline by 15N‑NMR. Its antiproliferative and cytotoxic effects have been tested on human Ewingʼs sarcoma MHH‑ES‑1 and breast cancer MT-3 cell lines, using cyclophosphamide and vincristine as reference controls. A Coulter counter was used to determine viable cell numbers, followed by the application of the tetrazolium compound MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxy phenyl)- 2-(4-sulfophenyl)-2H-tetrazolium] an inner salt. A colorimetric method was employed to evaluate cell viability in this cytotoxic assay. Micromolar concentrations of mitraphylline (5 µM to 40 µM) inhibited the growth of both cell lines in a dosedependent manner. The IC50 ± SE values were 17.15 ± 0.82 µM for MHH‑ES‑1 and 11.80 ± 1.03 µM for MT-3 for 30 hours, smaller than those obtained for the reference compounds. This action suggests that the pentacyclic oxindole alkaloid mitraphylline might be a new promising agent in the treatment of both human sarcoma and breast cancer.