Understanding neutralization is particularly relevant to an appreciation of the interaction between a virus and its antibody-synthesizing host since it is likely that viruses and the antibody system have evolved in response to reciprocally imposed selective pressures. Neutralization of viruses which only infect non-antibody-synthesizing hosts, while of considerable interest from of points of view is de facto without any such evolutionary signifi- a number cance. In this second category are viruses of plants, invertebrates, vertebrates below fish in the evolutionary scale which do not...