Integrins of the beta 1 family were isolated from human smooth muscle. SDS-PAGE analysis and subsequent immunoblotting demonstrated that integrin samples contain a protein immunologically related to beta 1 integrin subunit with the previously undescribed apparent molecular mass 205 kD. One-dimensional peptide mapping showed that the 205 kD protein is not a novel beta 1 related integrin subunit, but a beta 1 integrin subunit dimer. After reduction the major part of the beta 1 immunoreactive material migrated from the 205 kD to 130 kD region, indicating that beta 1 integrin subunit dimers were formed via disulfide bonds. When electrophoretically pure beta 1 monomer and dimer forms were analized it was found that during SDS-PAGE about 30% of beta 1 integrin subunit monomers were organized into dimers while approximately 70% of the beta 1 dimer form was partly disrupted into monomers. It was suggested that this steady-state process is a result of a reversible reaction between intra- and intermolecular disulfide bonds. Possible in vivo dimerization of integrins via disulfide bonds is discussed.