The success of gene therapy depends on the choice of a suitable vector that is biocompatible and efficient in delivering therapeutic DNA into disease cells. After more than two decades, such an ideal vector is still a wish. Viral vectors though naturally evolved to transfect cells are immunogenic. As alternatives, non-viral vectors such as polyethyleneimine have been exploited. We decided to investigate the in-vitro cytotoxicity of branched polyethyleneimine 800D, 25kD and linear 20kD on HeLa and Vero cells. At exponential phase, cells were exposed to polymers at concentration range of 0.5 to 1000mg/ml. Cells were MTT assayed after 24, 48 and 72hours for viability (IC50). Linear PEI was less toxic than the branched PEI in both cells. The IC50 (mg/ml) values (Mean