By immunoelectron microscopy the podocyte foot processes of the rat and human kidney have been shown to contain three major proteins of the contractile apparatus in muscle, i.e., actin, myosin, and the Z-line protein, alpha-actinin. Gel electrophoresis and immunoblot analysis of isolated glomeruli suggests that these proteins constitute an important part of the total glomerular protein contents. In the chicken kidney, the plasmalemmal portion of the foot processes that abuts the glomerular basement membrane was specifically labeled with antibodies against chicken gizzard vinculin and talin, two proteins thought to be important for the linkage of actin filaments to the lipid bilayer and to the receptor for fibronectin and laminin. Such a linkage may not only be important for the attachment of actin filaments to the plasma membrane, but could also be of functional significance for restricting the fibronectin-laminin receptor in its lateral diffusion in the plane of the lipid bilayer and to localize it at the basis of the podocytic foot processes. Assuming that actin, myosin, and alpha-actinin are arranged in a way that would allow the foot processes to generate contractile force this filament system might help the glomerular capillaries to resist the high intraluminal hydrostatic pressure as well as to actively modify the surface area for filtration. Vimentin and tubulin, the main protein subunits of intermediate filaments and microtubules, respectively, were confined to the podocyte cell body and the major processes but were virtually absent from the foot processes. This suggests that both proteins and their polymers are not important for the structure and function of the foot processes.