Aim. In the Mediterranean countries, several described medicinal plants are derived from Lebanon. According to Tohme et al. there are 2597 species in Lebanon. More than fifty two percent are endemic to Lebanon. In this paper we show that the ethanol fraction of the stem of Berberis libanotica is able to inhibit the viability of HTLV-1 positive (HuT-102) and HTLV-1 negative (CEM) cell lines of malignant T-cell leukemia. Methods. After traditional maceration to extract the ethanol fraction from Berberis libanotica stem, the in vitro viability was assayed. Results. The results suggest that Berberis libanotica (a Lebanese medicinal plant) contains a substantial amount of secondary metabolites such as alkaloids powerful in inhibiting the viability of HuT-102 and CEM cell lines. Conclusion. The obtained results demonstrate a novel anticancer property of Berberis libanotica stem extracts, in addition to the previously reported anti-inflammatory activity.