The surface of Rubella virus contains the glycoproteins E1 and E2. The E1 protein induces neutralizing antibodies and has been implicated in the process of recognition of cellular receptors. To gain information on the structural organization of the E1 protein we have analyzed the disulfide bonds present within this molecule. The reactivity of the protein with radioactively labeled iodoacetic acid indicates that all 20 cysteine residues present in the ectodomain of the E1 protein are involved in disulfide formation. E1 protein was purified by preparative SDS-PAGE under nonreducing conditions from virus particles grown in tissue culture in the presence of [35S]cysteine. The purified protein was digested with a number of proteases followed by reversed phase high-performance liquid chromatography (HPLC). [35S]cysteine-containing peptides were identified and characterized by N-terminal amino acid sequence determination. These analyses identified the following eight disulfide bridges: C(1)-C(2); C(3)-C(15); C(6)-C(7); C(9)-C(10); C(11)-C(12); C(13)-C(14); C(17)-C(18); and C(19)-C(20). The two disulfide bridges formed by the residues C(4), C(5), C(8), and C(16) have not been identified with certainty, but a likely organization can be derived. The data obtained are discussed in the context of a possible structural and functional organization of the E1 protein.