Tenascin is a large hexameric extracellular matrix protein. Each subunit consists of a linear array of the following structural domains: an N-terminal cysteine-rich region and heptad repeats both involved in the oligomerization, followed by EGF-like repeats, fibronectin type III domains and a C-terminal globular domain homologous to fibrinogens. Depending on the species, the number of EGF-like repeats as well as of the fibronectin type III repeats differs. A model proposing a new nomenclature for the numbering of the fibronectin type III repeats is presented. It enables the assignment of corresponding repeats from different species and shows that for human tenascin several more alternatively spliced fibronectin type III repeats have been described than for chicken or mouse tenascin. Tenascin is generally classified as an anti-adhesive protein, since many cells do not adhere to tenascin or if they adhere they do not spread. The addition of tenascin can also lead to the loss of focal contacts in well spread cells. On the other hand growth cones adhere well to tenascin and a tenascin substrate can support neurite extension. Using recombinant fragments of tenascin we could identify an adhesive as well as an anti-adhesive region within one tenascin subunit. The major interest in tenascin was prompted by its interesting tissue distribution. Tenascin is often transiently expressed during organogenesis, tissue remodeling, cell migration and the development of the nervous system. We thus investigated possibilities to regulate tenascin expression in primary chick embryo fibroblast cultures.(ABSTRACT TRUNCATED AT 250 WORDS)