Oldenlandia diffusa is traditionally prescribed in the treatment of a number of cancers and studies suggest that it exerts a cytotoxic action specific to cancer cells. To further investigate this suggested action, the effect(s) of Oldenlandia diffusa on leukaemic cells (HL60) and stimulated and unstimulated human blood lymphocytes (PBLs) was investigated. For the HL60s, cell growth, apoptotic induction, alterations in cell cycle characteristics and genotoxicity were investigated. For the PBLs, apoptotic induction and alterations in cell cycle characteristics were investigated. Preliminary chemical analysis to identify the cytotoxic constituents of Oldenlandia diffusa was also carried out. Results showed that Oldenlandia diffusa significantly inhibited the growth of the HL60s and induced apoptosis in a cell cycle-independent fashion, possibly through the induction of genotoxic damage. Oldenlandia diffusa did not induce apoptosis in the PBLs however progression through the cell cycle was not evident (in stimulated PBLs) suggesting some degree of cytotoxicity. Preliminary chemical analysis indicated that a number of compounds appear to be responsible for the cytotoxic action of Oldenlandia diffusa. These results indicate that HL60s are more sensitive to Oldenlandia diffusa than stimulated PBLs and thus support a cytotoxic action for Oldenlandia diffusa that has some degree of specificity.