Tenascin (TN) is a high-molecular-mass oligomeric glycoprotein of the extracellular matrix (ECM) endowed with a programmed expression in embryonic life and a neo-expression in the interstitium of some malignancies. Using monoclonal antibodies (MAbs) which identify human tenascin, we have conducted an extensive immunohistochemical analysis of TN expression in normal fetal and adult human tissues as well as in a wide variety of human tumors. Results of this study demonstrate that TN (1) is detectable in embryonic and fetal tissues at least from the 10th week of gestation; (2) is present in the interstitium of a variety of adult tissues of different embryonic origin; (3) may be neo-expressed in the stroma of benign and malignant tumors; (4) has the ability to accumulate in a highly variable manner in the ECM of tumors of the same and of different histotypes.