By sequencing cDNA clones which cover the complete coding region of human tenascin (TN), we have established its primary structure. This confirms that, as in the case of chicken, TN is mainly made up of three groups of sequences with a high homology to Epidermal Growth Factor (EGF) fibronectin (FN) type III repeat and fibrinogen. Furthermore, we have determined the amino-terminal sequence of the mature peptide directly on purified TN. The main differences with respect to the chicken TN molecule are that in the human there are 14 and half EGF-like repeats compared to 13 and half in the chicken and that, as previously reported, there are 15 FN-like repeats compared to 11 in the chicken. By Polymerase Chain Reaction (PCR) amplification we have also studied the different splicing patterns of the TN pre-mRNA in cultured cells. The results show the presence of at least four different isoforms containing different numbers of FN-like type III repeats. Using purified human TN as immunogen, we have obtained numerous monoclonal antibodies (Mabs) to TN. By screening a human melanoma cDNA library in the expression vector lambda gt11 with these Mabs and subsequently sequencing the insert of the positive clones, we have been able to localize, within the TN molecule, the epitopes recognized by two of these Mabs: BC-4, which recognizes an epitope within the EGF-like sequence and BC-2 which recognizes an epitope within the FN like type III repeats whose expression is regulated by alternative splicing of the TN pre-mRNA. Thus, while the Mab BC-4 may be useful in studies on TN distribution (since it recognizes all different TN isoforms) BC-2 may be useful in the study of the expression of particular TN isoforms generated by the alternative splicing of the TN primary transcript.