It was found that mononuclear cells in the lung parenchyma of mice C57BL/6 after intratracheal administration of bleomycin demonstrated mesenchymal phenotype and express surface CD44, CD73, CD90, CD106, but did not express CD31, CD34, CD45. Bleomycin enhances the differentiation of MSCs in the direction of fibroblast cells in vitro and does not change the activity of adipogenic differentiation, at the same time chondrogenesis and osteogenesis are inhibited. Our results demonstrate that intranasal inoculation of pegylated hyaluronidase (pegHYAL), blocks bleomycin-induced lung injury and fibrosis. In the acute stage of inflammation (day 7) pegHYAL reduces the number of hematopoietic stem cells in the lungs (HSCs). PegHYAL reduces the amount of CD45+ pan-hematopoietic cells, CFU-F and MSCs in the period of active deposition of collagen fibers in the lung (day 21). Antifibrotic effect of pegHYAL surpasses activity of HYAL. These data suggest that pegHYAL is a new and promising tool for the treatment of pulmonary fibrosis. This book is for the specialists working in the field of pathophysiology, pulmonology, cell biology, pharmacology.