Mesenchymal stem cells (MSCs) have been used in hematopoietic stem cell transplantation for years. However, the safety of MSCs applied in various types of hematologic malignancy has not been comprehensively explored. In the present study, the effects of human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) on six representative hematologic malignancy cell lines were explored, including leukemia, multiple myeloma and lymphoma cells. Direct and indirect co-culture models were established, and cell proliferation was assessed by carboxyfluorescein diacetate succinimidyl ester staining. A cytometric bead array cytokine kit was used to quantify cytokines. The expression of interleukin (IL)-6 receptor elements on tumor cells was detected by reverse transcription-polymerase chain reaction and flow cytometry, and the effects of exogenous IL-6 on cell proliferation were determined using a Cell Counting kit-8 assay. The results demonstrated that hUC-MSCs inhibited the proliferation of most of the cell lines examined (THP-1, HL-60, K562 and RPMI-8226), but promoted the proliferation of Raji cells. In addition, hUC-MSCs secreted abundant IL-6, promoted the secretion of IL-10 by RPMI-8226 and Raji cells, and inhibited the secretion of tumor necrosis factor-a by THP-1 cells. These data indicate a varied effect of hUC-MSCs on various types of hematologic malignancy, including distinct mechanisms of cell-to-cell contact and cytokines. Researchers applying hUC-MSCs in lymphoma should be aware of a potential tumor growth-promoting effect.