Myofibroblasts play a key role in wound closure but their origin is poorly understood. To investigate whether fibrocytes contribute to myofibroblast population, we examined the phenotype of fibrocytes and myofibroblasts present in the wounded skin of BALB/c mice. During wound healing, there was a marked increase in the number of cells expressing the myofibroblast marker alpha-smooth muscle actin in the granulation tissue. Between 4 and 7 days post-wounding, more than 50% of these cells also expressed the CD13 antigen. CD13(+)/collagen I+ fibrocytes could be isolated at an early stage of the healing process from digested fragments of wounded tissue by fluorescence-activated cell sorting. Like authentic fibrocytes, these cells were also CD45(+)/CD34(+)/CD14-. Between 4 and 7 days post-injury, 61.4% of the isolated fibrocytes were found to express alpha-smooth muscle actin gene and protein. We repeated similar experiments in female mice that had received a male whole bone marrow transplant after total body irradiation. By in situ hybridization, we identified the Y chromosome in the nuclei of the majority of fibrocytes isolated from the wounded tissue of these animals. Our data indicate that circulating fibrocytes contribute to the myofibroblast population in the wounded skin and that they originate from the bone marrow.