Objective To compare the morphological characters and collagen synthesis between circulating fibrocytes and fibroblasts. Methods Circulating fibrocytes were isolated from human peripheral blood by centrifugation over FicollPaque and cultured in vitro. Then the collagen secreted into the medium was detected by the measurement of the hydroxyproline concentration with enzymolysis, and the collagen accumulated on the cell layer was determined by the optical density at 540nm of picrosirius red staining. The contents of collagen were assessed by Western blotting. Results The morphological characteristics of circulating fibrocytes were varying when cultured in vitro and the collagen types Ⅰ and Ⅲ were synthesized and secreted into the medium.Conclusion Circulating fibrocytes can differentiate into fi