Objective To investigate the effect of mesenchymal stem cells (MSCs) on subcutaneous xenograft tumors in mice with Lewis lung cancer. Methods MSCs isolated from bone marrow of C57BL/6 mice were made into single cell suspension and were cultured in vitro. The cells of the 4th to 5th passage were used for the subsequent experiments. Fifty six C57BL/6 mice were inoculated subcutaneously with Lewis lung cancer cells, and were grouped into Group D0 (MSCs were given simultaneously with inoculation)and Group D10(MSCs were given 10 d after inoculation). Group D0 included three subgroups (n=8): Group 1 with inoculation of tumor cells, Group 2 with inoculation of tumor cells and MSCs, and Group 3 with inoculation of tumor cells and tail intravenous injection of MSCs. Group D10 included four groups: Group 4 with inoculation of tumor cells and injection of MSCs in tumors, Group 5 with equivalent PBS (the control of Group 4), Group 6 with inoculation of tumor cells and tail intravenous injection of MSCs, and Group 7 with equivalent PBS (the control of Group 6). The time of tumor formation and the volume of tumor were observed and compared among the groups. ResultsIn Group D0, earlier onset of tumor development was observed in Group 2 as compared to Group 1 and Group 3 (P<0.05), while there was no significant difference on the volume of tumor in the three groups (P>0.05). In Group D10, the volume of tumors were larger in Group 4 compared to the control (P<0.05), while there was no significant difference on the volume of tumors between Group 6 and the control (P>0.05). Conclusion Inoculating mixture of MSCs and Lewis lung cancer cells accelerates tumor formation,and injection of MSCs in tumors stimulates the growth of tumors.