We have studied the functions of contractile proteins in Acanthamoeba by a combination of structural, biochemical and physiological approaches. We used electron microscopy and image processing to determine the three-dimensional structure of actin and the orientation of the molecule in the actin filament. We measured the rate constants for actin filament elongation and re-evaluated the effect of MgCl2 on the filament nucleation process. In Acanthamoeba actin polymerization is regulated, at least in part, by profilin, which binds to actin monomers, and by capping protein, which both nucleates polymerization and blocks monomer addition at the 'barbed' end of the filament. To test for physiological functions of myosin-II, we produced a monoclonal antibody that inhibits the actin-activated ATPase. When microinjected into living cells, this active-site-specific antibody inhibits amoeboid locomotion. We expect that similar experiments can be used to test for the physiological functions of the other components of the Acanthamoeba contractile system.