Monoclonal antibody 9G1 (mAb 9G1) is reactive to the wound epithelium of axolotl larvae and therefore provided the opportunity to examine the interaction between the wound epithelium, nerves, and blastemal mesenchyme during axolotl limb regeneration. In unamputated limbs, mAb 9G1 is reactive to most or all cells of the dermis, skeletal elements, blood vessels, and nerves, to a few unidentified cells in muscle, and to none in epidermis. During regeneration of axolotl limbs, mAb 9G1 reacts strongly to an intracellular antigen of the blastemal mesenchyme and of the distal-most portion of the wound epithelium, the so-called apical epithelial cap (AEC). Because this thickened wound epithelium of regenerating amphibian limbs has been suggested as functioning in a manner similar to the apical ectodermal ridge (AER) of embryonic limb buds, it was of interest to further examine the reactivity of mAb 9G1 during various stages of regeneration. Whether mAb 9G1 reactivity in the AEC depended on mesenchyme and/or nerves was also tested. Monoclonal antibody 9G1 reactivity appears in the AEC of regenerating limbs prior to outgrowth of the blastema and persists throughout blastemal stages. Apical epithelial cap reactivity to mAb 9G1 is nerve dependent during early stages of blastema development and becomes nerve-independent at later stages. When epithelium-free blastemal mesenchyme is grafted onto injured flank musculature, ectopic limb regeneration occurs and the AEC derived from flank epidermis exhibits mAb 9G1 reactivity. These results show that a mAb 9G1 reactive AEC is characteristic of regenerating limbs and that expression of the 9G1 antigen by the AEC is dependent upon underlying blastemal mesenchyme and nerves.