Objective: The umbilical cord and placenta are extraembrionic tissues of particular interest for regenerative medicine. Although traditionally discarded upon birth, the placenta and umbilical cord are now regarded as a valuable source of cells with stem cell-like plasticity. Umbilical cord mesenchymal stem cells (MSCs) have been shown to inhibit breast cancer cell growth but it is not known whether this effect is specific to only breast cancer cells. Materials and Methods: We compared the effects of human Wharton’s jelly stem cell (hWJSC) and amniotic stem cell (hASC) co-cultures on human myelogenous leukemia cell line K562. The cells were co-cultured with either hWJSC (confluence 60%) or hASC for 24-72 hours and changes in cell morphology, proliferation, and cell death were studied. Results: All three time course co-cultures of cancer cells with hWJSCs showed growth inhibition with cell death (`10-50%), accompanied with cell shrinkage and blebbing, while growth assay of cancer cell co-cultures with hASCs showed no considerable growth inhibition (by 4-9%). Cell cycle assays showed increases in G1 phase for all three time course co-cultures. Conclusion: Suggestive, hWJSCs possess tumor inhibitory properties that are not specific to breast cancer cells alone and these effects are mediated via agents in their growth media.