Caldesmon is a major actin and calmodulin binding protein of smooth muscle. It is located in thin filaments and appears to play a role in the regulation of actin-myosin interaction. The binding of caldesmon to F-actin results in the inhibition of ATPase activity in smooth and skeletal muscle. The combined action of caldesmon and F-actin is itself inhibited by calmodulin in the presence of calcium ions. This antibody is a very useful marker for distinguishing between peripheral fibroblasts and myofibroblasts in colorectal tumours.