A number of different kinases, such as cGMP-dependent protein kinase, and C-kinase will be used to phosphorylate myosin light chain kinase isolated from platelets and bovine trachea. The site(s) phosphorylated by the various kinases will be identified following phosphorylation of myosin kinase in the presence and absence of bound calmodulin. The effect of phosphorylation on myosin kinase activity and on the ability of myosin kinase to bind calmodulin before and after phosphorylation will be studied.