We have described two types of oligosaccharide modification influencing the antigenicity of the herpes simplex virus type 1 (HSV-1)-specified glycoprotein C (gC-1). First, the expression of several epitopes belonging to antigenic site II of gC-1 is dependent on the peripheral galactose of N-linked oligosaccharides. We have also shown that treatment of HSV-1-infected cells with 5-n-propyl-2'-deoxyuridine (PdU) under certain circumstances results in other modifications of peripheral carbohydrate determinants, which are associated with increased antigenic activity of gC-1. In the present study we have mapped and characterized the epitopes susceptible to PdU induction by analysing the reactivity to a number of monoclonal antibodies defining several epitopes of antigenic sites I and II. The results indicate that the strict galactose dependence of epitopes and the PdU-induced increase of antigenic activity are independent and unrelated phenomena. Thus, we identified galactose-dependent epitopes that were not PdU-inducible and vice versa, and some epitopes were both galactose-dependent and PdU-inducible. The results support a model where PdU treatment blocks synthesis of an antigen-masking carbohydrate determinant. In addition, PdU treatment of HSV-1-infected cells seemed to increase the antigenic activity of other HSV-1 glycoproteins.