Monoclonal antibodies (mAb) utilized in regeneration studies to date identify antigens that are up-regulated in the blastema. We obtained a monoclonal antibody, designated ST1 (Stump 1), that is reactive to an extracellular matrix (ECM) antigen exhibiting the opposite distribution; ST1 is an abundant antigen of the limb stump soft tissues but is absent from within the blastema. The border between abundance and absence of mAb ST1 reactivity was sharp and extended as a concavity into the stump. This distinct dichotomy led to further studies relevant to understanding how this extracellular matrix antigen is modulated during regeneration. mAb ST1 reactivity decreased in the internal tissues at the distal end of the limb prior to blastema formation and remained absent until the onset of differentiation. The initial decrease in mAb ST1 reactivity was dependent on the combined effects of injury and the wound epithelium but was nerve independent. At blastema stages of regeneration, the distribution of tenascin, ascertained by mAb MT1 reactivity, closely matched the area without reactivity to mAb ST1. The spatial and temporal distribution of the ST1 antigen in unamputated limbs and during regeneration did not correspond to any previously described ECM component.