Spectrin is an actin binding protein that is a major component of the plasma membrane skeleton. Spectrins function as membrane organizers and stabilizers by forming dimers, tetramers and higher polymers. Spectrin Alpha and spectrin Beta are present in erythrocytes, whereas spectrin Alpha II (also designated fodrin Alpha) and spectrin Beta I (also designated fodrin Beta) are present in other somatic cells. The spectrin tetramers in erythrocytes act as barriers to lateral diffusion, but spectrin dimers seem to lack this function. Spectrin Beta III is highly homologous to both spectrin Beta I and spectrin Beta II. Western blot analysis shows that spectrin Beta III migrates at a higher molecular mass than predicted in the kidney. Spectrin Beta III is highly expressed in brain, kidney, pancreas, and liver, and at lower levels in lung and placenta. Specifically, spectrin Beta III constitutes a major component of the Golgi and vesicular membrane skeletons.