Two distinct patterns of neutralization were identified by comparing the neutralization curves of monoclonal antibodies (MAbs) directed at the two surface proteins, VP4 and VP7, of rhesus rotavirus. VP7-specific MAbs were able to neutralize virus efficiently, and slight increases in antibody concentration resulted in a sharp decline in infectivity. On the other hand, MAbs to VP4 proved much less efficient at neutralizing rhesus rotavirus, and the fraction of infectious virus decreased gradually throughout a wide range of antibody concentrations. MAbs directed at VP8*, the smaller trypsin cleavage fragment of VP4, were shown to efficiently prevent binding of radiolabeled virions to MA104 cell monolayers, to an extent and at concentrations comparable to those required for neutralization of infectivity. Conversely, MAbs recognizing VP7 or the larger VP4 trypsin cleavage product, VP5*, showed little or no inhibitory effect on virus binding to cells. All MAbs studied were able to neutralize rotavirus that was already bound to the surface of cells. The MAbs directed at VP8*, but not those recognizing VP5* or VP7, were shown to mediate release of radiolabeled virus from the surface of the cells. With MAbs directed at VP7, papain digestion of virus-bound antibody molecules led to an almost complete recovery of infectivity. Neutralization could be fully restored by incubation of virus-Fab complexes with anti-mouse immunoglobulin G antiserum. Neutralization with MAbs directed at VP8* proved insensitive to digestion with papain as well as to the addition of anti-immunoglobulin antibodies.