Tenascin‐C is a modular glycoprotein composed of domains of amino acid repeats. All forms of tenascin‐C have eight constant fibronectin type III repeats, but additional fibronectin type III repeats can be spliced into a variable domain found between the fifth and sixth constant repeats. Four extra repeats, named A, B, C and D, have been examined previously. Here, we have used in situ hybridization to determine the tissue origins of the novel AD1 and AD2 repeats. In the embryonic‐day‐10 chicken embryo, transcripts encoding the AD2 repeat are limited to the tips of lung bronchioles and the base of feather buds. In contrast the AD1 hybridization signal was widespread. Quantitative in situ hybridization reveals AD1‐containing transcripts represent up to 85% of the total tenascin‐C mRNA in some tissues (developing bone), and are undetectable in others (e.g. radial glia). Avian and human tumor cell lines were examined for the expression of the AD1 repeat using the reverse transcriptase polymerase chain reaction (RT‐PCR). Transcripts encoding six different tenascin‐C splice variants incorporating the AD1 repeat were found in the fibrosarcoma cell line, QT6. Many human tumor cells, including malignant melanoma and ductal breast carcinoma, were positive for AD1 tenascin‐C expression. In addition, we found evidence of AD1 tenascin‐C expression in samples of excised human tumors. Our results show that a novel variant may be a major part of the tenascin‐C of the embryonic extracellular matrix, and may also be found in the stroma surrounding some human tumors.