Factors regulating the attachment and directional migration of a regenerating epidermis in wound healing are poorly understood. In studies of guinea pig 4-mm skin wounds, left uncovered for 1-28 days, biopsied and processed for 1-micrometer section and immunofluorescence, the epidermis migrated over an irregular thickened provisional matrix containing fibrin and fibronectin. The provisional matrix lacked two major components of normal basement membrane, laminin and type IV collagen, which can mediate tenacious epithelial attachment to plastic in vitro and may limit epidermal cell migration in vivo. Upon completion of wound reepithelialization at 7-9 days after wounding, the basement membrane zone lost its thickened appearance, fibronectin and fibrinogen disappeared, and type IV collagen and laminin reappeared. Although these findings do not prove that epidermal cell migration during reepithelialization required a fibrin and fibronectin matrix, they demonstrate that epidermal cells do move over such a substratum during in vivo wound repair.