• Mesodermal growth factor (MGF) from mouse submaxillary glands was tested in vivo for stimulating effects on corneal wounds in rabbits. Intrastromal injection of 5 μ g of MGF induced widespread fibroblast activity and stromal cell division, and markedly stimulated stromal healing. At high doses (> 25 μg), corneal destruction was indicated by extensive necrosis and perforation. When low doses (1 to 5 μg) of MGF were applied to the lip of nonperforating knife wounds of the cornea, three major differences were noted between control and experimental wounds. In wounds treated with MGF, the depth of stromal healing was greater, as was the intensity of the fibroblast activity, and the width and depth of the epithelial plug were significantly decreased. These results establish that MGF is an effective growth-stimulating agent in vivo and that the initial stages of corneal wound healing may be accelerated in vivo.