Impact of low doses of active oxygen forms (AOFs) on the paracrine activity of mesenchimal stromal cells (MSCs) was studied. Photodynamic treatment (PDT) was shown to be a method for controlled generation of intracellular AOFs. Active oxygen forms generated at a dose of 0.25J/cm² do not impact significantly the MSCs mitochondrial activity or viability and can be recognized as regulatory. This was the first discovery that low-intensity PDT modulates substantially the MSCs paracrine activity which was manifested by an increased secretion of proinflammatory cytokines (IL-6, IL-8), vascular endothelium growth factor (VEGF), and suppressed secretion of the transforming growth factor (TGFβ). While expression of intercellular adhesion molecule ICAM-1 increases and of Thy-1 antigen (a common MSCs marker) decreases, no changes occur to expression of chemokine receptor CXCR4 or other adhesion molecules (H-CAM, MCAM and VCAM-1). Our data make it clear that low-dose PDT is the most important regulator of the MSCs function. Key words: active oxygen forms, mesenchimal stromal cells, paracrine activity, photodynamic exposure.