Follow-up studies of antibody-titres in patients suffering from recidivations of herpes labialis and progenitalis treated with herpes virus vaccine inactivated by heat. It was stated by several authors that patients suffering from recidivations of herpes labialis and progenitalis can be successfully treated with a herpes virus vaccine inactivated by heat (Lupidon G/H). Nothing is known, however, about the mechanisms diminishing recurrences. We have studied 11 patients and 3 control persons whether there were changes in specific antibody titres after repeated vaccinations. Skin reactions following intradermal injections were also tested. Serum samples studied over a period of 8 weeks until 9 months showed that the antibody titres found by complement-fixation and neutralization tests remained practically constant. Skin tests were always negative. Therefore it merits further study to find out the mode of action of this clinically successful treatment.