An immunochemical analysis of the hemagglutinin (VP4) of the simian rotavirus SA11 was performed to better understand the structure and function of this molecule. Following immunization of mice with double-shelled virus particles and VP4-enriched fractions from CsCl gradients, a battery of anti-SA11 hybridomas was generated. A total of 13 clones secreting high levels of anti-VP4 monoclonal antibody (MAb) was characterized and compared with two cross-reactive anti-VP4 MAbs generated against heterologous rhesus (RRV) and porcine (OSU) rotavirus strains. These cross-reactive MAbs effectively neutralized SA11 infectivity in vitro. The epitopes recognized by these 15 MAbs were grouped into six antigenic sites on the SA11 hemagglutinin. These sites were identified following analysis of the MAbs by using a simple competitive binding enzyme-linked immunosorbent assay (ELISA) and biological assays. Three of the antigenic sites were involved in neutralization of virus infectivity in vitro. All the MAbs with neutralization activity and two nonneutralizing MAbs were able to inhibit viral hemagglutination of human erythrocytes. Competitive binding ELISA data showed a positive cooperative binding effect with some pairs of the anti-VP4 MAbs, apparently due to a conformational change induced by the binding of the first MAb. Some of the MAbs also bound better to trypsin-treated virus than to non-trypsin-treated virus. A topographic map for VP4 is proposed on the basis of the observed properties of each antigenic site.