The experiments described in this paper were designed to assess the role of the various virus glycoproteins of pseudorabies virus (PrV) in eliciting the production of neutralizing antibodies during the normal course of infection of swine. They also address the question of the degree of antigenic variation within each glycoprotein between different virus isolates. The results show the following: Antigenic variation between strains of PrV isolated from different geographic areas are readily detectable; antigenic differences between strains isolated from the same geographic area are less common. No antigenic drift in glycoprotein gII was observed. Glycoprotein gIII and, to some extent, also glycoprotein gI showed a high level of antigenic drift. The neutralizing activity of pooled convalescent sera of swine is not directed against glycoprotein gI. A large part of the neutralizing activity of pooled convalescent sera of swine is directed against glycoprotein gIII.