A new leukocyte subpopulation, fibrocytes, has been identified as circulating cells with fibroblast-like properties that mediate tissue repair. These cells, characterized by a distinctive phenotype (collagen+/vimentin+/CD34+), enter sites of tissue injury and contribute to scar formation. Unlike previously thought, fibrocytes do not solely arise from surrounding connective tissue but may originate from blood. The study used a combination of immunofluorescence, functional analyses, and electron microscopy to isolate and characterize these cells. Fibrocytes were found to be present in connective tissue scars and to share features with both leukocytes and connective tissue cells. They express fibroblast markers such as collagen I, vimentin, and fibronectin, as well as leukocyte common antigen CD45 and hematopoietic stem cell marker CD34. Bone marrow chimera studies suggested that fibrocytes may originate from radioresistant bone marrow stromal elements or other mesenchymal cell progenitors. The presence of fibrocytes in wound chambers and scar tissue indicates their role in tissue repair. These cells may also contribute to pathological fibrotic responses. The study provides direct evidence for the involvement of fibrocytes in the host repair response to tissue injury.A new leukocyte subpopulation, fibrocytes, has been identified as circulating cells with fibroblast-like properties that mediate tissue repair. These cells, characterized by a distinctive phenotype (collagen+/vimentin+/CD34+), enter sites of tissue injury and contribute to scar formation. Unlike previously thought, fibrocytes do not solely arise from surrounding connective tissue but may originate from blood. The study used a combination of immunofluorescence, functional analyses, and electron microscopy to isolate and characterize these cells. Fibrocytes were found to be present in connective tissue scars and to share features with both leukocytes and connective tissue cells. They express fibroblast markers such as collagen I, vimentin, and fibronectin, as well as leukocyte common antigen CD45 and hematopoietic stem cell marker CD34. Bone marrow chimera studies suggested that fibrocytes may originate from radioresistant bone marrow stromal elements or other mesenchymal cell progenitors. The presence of fibrocytes in wound chambers and scar tissue indicates their role in tissue repair. These cells may also contribute to pathological fibrotic responses. The study provides direct evidence for the involvement of fibrocytes in the host repair response to tissue injury.