The expression and distribution of cytokeratins 8, 18, 19, pan-cytokeratin, vimentin, type IV collagen, laminin and fibronectin were investigated immunohistochemically in the Wolffian and Müllerian ducts and perimüllerian mesenchyme of human male fetuses ranging from 8 to 12 weeks of gestation. The epithelial cells of both genital ducts, the coelomic epithelium and the mesenchymal perimüllerian cells coexpressed cytokeratins 18, 19, pancytokeratin and vimentin. Type IV collagen, laminin, and fibronectin were detected in the basement membranes of both genital ducts, the coelomic epithelium and in the interstitium of the perimüllerian mesenchyme. The coelomic basement membrane adjacent to the müllerian duct showed interruptions and perimüllerian cells were in contact with the coelomic epithelium. The cytoskeletal immunophenotype of perimüllerian cells and their relationship with the coelomic epithelium suggested their coelomic origin. During the 12th week of gestation, müllerian duct began to regress with its basement membrane detaching from the surrounding mesenchyme. This coincided with the disappearance of vimentin from the ductal epithelial cells, as well as type IV collagen, laminin and fibronectin from perimüllerian mesenchyme. Thus müllerian ductal basement membrane dissolved. These findings indicate that the human müllerian duct regression is associated with changes in the interactions between Müllerian ductal and perimüllerian mesenchymal cells.