In recent years, the Mn-doped ZnS quantum dots (QDs) have been found to be excellent materials for fluorescence imaging of cells and for the detection of molecules such as folic acid, acetone etc. The fluorescence efficiency of this particular probe is due to the changes occurred in the electronic states of Mn and ZnS by means of doping. However, this report deals with the negative shades of Mn doping onto ZnS QDs as the formed Mn-ZnS nanocomposite is investigated to produce the Mn-induced oxidative stress that can be lethal to the tested cells in vitro. For that, we first synthesized Mn2+-doped ZnS nanocrystals by the co-precipitation method and the physical characterization was performed by means of HRTEM, EDAX, FTIR, XRD and fluorescence spectroscopies. Following that, we tested for the toxicity effects of Mn-ZnS QDs formed at room temperature by the co-precipitation method with BRL 3A cells of hepatic tissue of rat origin. The formed QDs investigated to show significant levels of toxicity toward the cells at 100 mu g/mL concentration and over an exposure period of 16 h. Further, the cells also investigated to generate the reactive oxygen species which is further responsible for the fragmentation of DNA and reduction in mitochondrial transmembrane potentials. From the analysis of results, we believe that this report of its kind provides a preliminary piece of information toward the designing and applicability of Mn-containing compounds to biomedical sector due to its severe oxidative stress effect.