A monoclonal antibody (30.2) raised against Sindbis virus is able to precipitate both E1 and PE2 from [35S]methionine-labelled infected cells solubilized with non-ionic detergent. Addition of SDS to the lysate abolishes the precipitation of PE2 without affecting that of E1, thus demonstrating that the antibody is specific for E1. Other Sindbis E1-specific monoclonal antibodies (30.11 and 30.12) precipitate only E1, even from lysates containing only non-ionic detergent, and their presence in such a lysate prevents precipitation of PE2 by antibody 30.2. These data indicate that E1-PE2 complexes stable in the presence of non-ionic detergent can be precipitated as such by one antibody, but that binding of the other antibodies induces dissociation of E1 and PE2. Competition experiments using 125I-labelled antibodies indicate that all three antibodies bind to distinct antigenic sites on the E1 molecule. Antibodies 30.11 and 30.12 stimulate each other's binding in such experiments, which suggests that binding of either of these antibodies alters the conformation of E1 in such a way as to increase its affinity for the other, and at the same time to release PE2. Antibody 30.2 also enhances binding of the other two antibodies, but this stimulation is only weakly reciprocated.