For the sake of brevity, I shall confine this review to the inactivation of virus infectivity which is the direct result of combination with neutralizing antibody. This restriction is solely for convenience and is not intended as a judgement of the importance of neutralization versus inactivation involving secondary factors such as complement or cells which interact with antibodies. Neutralizing antibody is one of the main arms of protective immunity and is the type of immunity which usually results from vaccination, so it is somewhat surprising to find that we understand little of the interaction between neutralizing antibody and antigenic determinants of the virus, and how as a result infectivity is lost. In fact, the situation is worse than mere ignorance and it is probably not overstating the case to say that in the past the dogma of neutralization has been based on a logical fallacy.