This study was designed to investigate the effects of pituitary fibroblast growth factor (FGF), endothelial cell growth supplement (ECGS), and human serum albumin (HSA) on the mitotic indices of dedifferentiating stump cells in regenerating limbs deprived of a wound epidermis. These cells would normally contribute to blastema cell formation but have been prevented from doing so by the placement of a full thickness skin flap over the amputation site. FGF, ECGS, and HSA were released from copolymer implants inserted between full thickness skin flaps and tibia-fibula amputation sites in axolotls, Ambystoma mexicanum. The implants were plain or contained either FGF or ECGS or HSA. After 2 wk the mitotic indices of the area of dedifferentiation within these experimental limbs were determined and compared to the mitotic indices of normal regeneration blastemas. Histological analysis revealed that the technique of suturing the full thickness skin flaps into position prevented the formation of dermis-free areas that can permit, regeneration. Full thickness skin flaps alone, or with a plain implant, or with an HSA-impregnated implant completely inhibited blastema formation in axolotl hindlimbs. The mitotic indices of these animals showed significant decreases compared to mitotic indices of normal regeneration blastemas. The mitotic indices of animals treated with full thickness skin flaps and implants impregnated with either FGF or ECGS showed significant increases compared to those of normal regeneration blastemas. Precocious cartilage development was found in limbs which contained FGF implants.