Muscle fibers and myosin B threads shortened in neutral K2HgI4 solutionThe speed and extent of shortening of psoas muscle fibers depended on the concentration of the reagent.KI was shown to be the contracting agent, but its action was counterbalanced by an elongation process which could be eliminated by HgI2.Similar results were obtained with KSCN and HgI2.In addition to shortening the fibers, the reagents made them very elastic.Stretched fibers freed of reagent remained "set," and shortened again when treated with the reagent.Myosin B threads behaved like muscle fibers.