Current Topics in Cellular Regulation: Volume 27, Modulation by Covalent Modification is a compendium of papers dealing with the coordination, function, or control of cellular metabolism, particularly on modulation by covalent modification. One paper reviews the cyclic cascade model in metabolic regulation that shows the model's applicability to all covalent interconvertible enzyme systems, such as those modified by phosphorylation, ADP-ribosylation, carboxymethylation, acetylation, and sulfation. Another paper discusses the hypothesis that smooth muscle contraction is regulated by a calcium-dependent phosphorylation of the myosin molecule. Studies made by Sellers and Pato suggest that a phosphatase isolated from turkey gizzard muscle (known as smooth muscle phosphatase IV) can be related to an in vivo myosin phosphatase as the phosphatase IV binds well to both myosin and thiophosphorylated myosin. Other papers describe in situ regulation studies of Excherichia coli glutamine synthethase, the posttranslational processing of the insulin proreceptor, and selenium in cellular metabolism. The compendium is suitable for biochemists, microbiologists, and academicians whose works involve research in cell biology.