The minor outer capsid shell component σ1 is the reovirus cell attachment protein because (a) IgGs directed against σ1 prevent reovirus particle adsorption, (b) free protein σ1 present in lysates of infected cells is capable of adsorbing to cells, and (c) σ1 competes with reovirus particles for cell surface receptors. Competition experiments indicate that reovirus particles of all three serotypes attach to the same cell surface receptor. It is also shown that σ1 is located on the surface of reovirus particles in close juxtaposition to λ2, the major, if not the only, component of the 12 icosahedrally distributed reovirus core projections or spikes, which penetrate through the outer capsid shell to the reovirus particle surface.