Peripheral blood fibrocytes are a newly identified leukocyte subpopulation that displays fibroblast-like properties. These blood-borne cells can rapidly enter the site of injury at the same time as circulating inflammatory cells. We hypothesize that circulating fibrocytes represent an important source of fibroblasts for healing of extensive burn wounds where it may be difficult for fibroblasts to migrate from the edges of uninjured tissue. In this study we identified and quantified fibrocytes among the adherent cells cultured from human peripheral blood mononuclear cells (PBMC) obtained from 18 burn patients and 12 normal individuals, based on their ability to express type I collagen. Our results showed that adherent cells cultured from PBMC of burn patients differentiated to fibrocytes more efficiently than did those from normal individuals. The percentage of type I collagen-positive fibrocytes was significantly higher for patients than for controls (89.7 +/- 7.9% versus 69.9 +/- 14.7%, p < 0.001). This percentage was consistently higher for patients with a >/=30% total body surface area burn until 1 year, with the highest percentage appearing within 3 weeks of injury. A positive correlation was found between the levels of serum transforming growth factor-beta1 (TGF-beta1) and the percentage of fibrocytes developing in the cultures of PBMC derived from these patients. We also demonstrated that fibrocytes were derived from CD14(+) cells but not CD14(-) cells. Conditioned medium from CD14(-) cells was, however, required for fibrocyte differentiation, whereas direct contact between CD14(-) and CD14(+) cells was not necessary. Treatment of the cell cultures with TGF-beta1 enhanced the development of collagen-positive cells, whereas the inclusion of neutralizing anti-TGF-beta1 antibodies in the CD14(-) conditioned medium suppressed fibrocyte differentiation. These data suggest that the development of fibrocytes is up-regulated systemically in burn patients. Increased TGF-beta in serum stimulates the differentiation of the CD14(+) cell population in PBMC into collagen-producing cells that may be important in wound healing and scarring.