Neutralization of infectivity is the end result of a number of alternative or synergistic reactions between a virus and antibody directed against certain antigenic determinants on its surface. These reactions prevent either the attachment of virus to a particular host cell by steric hindrance, the entry of virus into the cell, or the uncoating of the virus particle to release the viral genome. In turn, the relative contribution of each of these to neutralization is dependent both on qualities of the viral antibodies (their relative affinities and immunoglobulin class) and specificity for relevant antigenic determinants (so-called critical sites). These considerations have been the subjects of several extensive reviews (Mandel 1979; Della-Porta and Westaway 1978).