Mouse L cell lines constitutively expressing glycoproteins B or D of herpes simplex virus type 1 (LTKgB and LTKgD respectively) were used to study the longevity of the immune response to these viral glycoproteins in mice. Two immunizations with the cell lines were necessary to induce a persisting antibody response (present for over 200 days). Only LTKgD induced a neutralizing at antibody response in mice and this also remained at high titres over 200 days after two inoculations. The presence in mice of precursor cytotoxic T lymphocytes specific for gB expressed in the L cells was also shown up to 270 days after immunization. Mice immunized with the cell lines showed an increased rate of virus clearance from the ear pinna, inoculation with LTKgD resulting in more clearance than LTKgB at 7 days post-immunization. This type of protection was reduced with time after inoculation, until by day 161 there was no significant difference in virus titres between immunized and control groups. However, LTKgD immunization protected against the establishment of latent infections in the ganglia of mice even up to 186 days post-inoculation.