The reovirus cell-attachment protein, which modulates tissue tropism and the nature of the antiviral immune response, is protein sigma 1. This protein is present in reovirus particles in the form of 12 tetramers anchored in the projections or spikes. It is formed in infected cells in very small amounts only. In order to produce larger amounts of it that are necessary for studying its properties in detail, we have cloned it into a mammalian expression vector system that consists of the very strong cowpox virus A-type inclusion body protein gene promoter inserted into the vaccinia virus genome. In cells infected with the resultant recombinant vaccina virus, protein sigma 1 is formed in large amounts (2-3 mg/10(9) cells). We have isolated the native protein and shown that it exists as tetramers that possess cell-binding activity, hemagglutinating activity, and functional epitopes recognized by both polyvalent antisera and monoclonal antibodies.