In this study, different concentrations of the methanolic extract of leaves of the plant Orthosiphon thymiflorus were subjected to cytotoxic activity study against Dalton Lymphoma Ascites (DLA) cells using the MTT assay. Percentage cell viability of cell lines were carried out by using Trypan blue dye exclusion technique MTT assay was used to evaluate the reduction of viability of cell cultures in the presence and absence of the extract. Cell viability was inhibited to different extents by different concentrations of the extract.