Methods for stimulating proliferation of cells in the epidermis and dermis of mammalian skin are disclosed. The methods comprise the step of administering to an area of the skin a composition comprising a therapeutically effective amount of human placental alkaline phosphatase, or an active derivative. The composition can be administered topically or by injection. The invention also provides a regimen for restoring or maintaining the strength and thickness of aging skin, comprising periodically administering a composition by topical application. The invention further provides methods for stimulating proliferation of cells in the epidermis and dermis of transplanted skin. A composition for topical application is also provided by the invention.