The tenascins are a family of extracellular matrix proteins postulated to have important developmental functions. Mice deficient in tenascinC (TNC) are normal. Deficiency of tenascinX (TNX) causes an EhlersDanloslike syndrome in humans, but not disease of heart or muscle where TNX is most prominently expressed. In human skin and epicardium, expression of TNX and TNC is overlapping, while in muscle and vasculature expression is contiguous, but nonoverlapping. To understand whether TNX and TNC share functions, we compared the adhesive and antiadhesive effects of purified TNX and TNC on L6E9 skeletal myoblasts. Myoblasts are exposed to TNX and TNC in vivo. but express neither. Both proteins supported binding of <5% of plated myoblasts, while fibronectin (FN) supported binding of >80%. To evaluate antiadhesive effects of TNX and TNC, inhibition of cell binding to FN was evaluated. When coated over FN in solid phase, both proteins inhibited binding in a dose dependent manner. When L6E9 cells in suspension were treated with TNX or TNC and washed before plating on FN, TNX inhibited binding much more potently than TNC. This suggests antiadhesion by TNX occurs through an effect at the cell surface, while TNC may act by binding to FN and steric interference with its cell surface receptors. Finally, based on the patterns of expression in vivo, we wondered whether TNX might directly regulate expression of TNC. Human skin and muscle fibroblasts were plated on uncoated plastic, TNXcoated plastic, or plastic followed by TNX addition after cell attachment. After 24 hours cell morphology was identical in the 3 conditions, but TNX stimulated TNC protein secretion by skin fibroblasts 550 fold, while it produced mild inhibition of TNC secretion by muscle fibroblasts. RNase protection analysis demonstrated changes in TNC protein secretion are due to changes in TNC mRNA level. Functional similarities between TNX and TNC and regulatory interactions may allow one tenascin family member to compensate for isolated deficiency of another.