<p>The effect of the microenvironment on the efficiency of anti-cancer therapy is one of the most discussed topics in modern oncology. In this study, we evaluated the effect of co-cultivation with human mesenchymal stem cells (MSCs) on the efficiency of DNA double-strand break (DSB) repair in human non-small cell lung cancer (NSCLC) A549 cell line exposed to X-ray radiation. In addition, the effect of co-cultivation of MSCs and A549 cells on the proliferative activity of non-irradiated NSCLC was also studied. To assess the efficiency of DNA DSB repair, we analyzed the quantitative yield of residual foci of DSB marker proteins (γH2AX and 53BP1) 24 h after irradiation with doses of 2, 4, and 6 Gy. The results showed that co-cultivation with MSCs did not affect the efficiency of DNA DSB repair induced by X-rays, as well as the proliferative activity of NSCLC cells.</p>