Since quite some time relapsing herpes simplex virus infections of the skin are clinically successfully treated with a heat-inactivated vaccine Lupidon G and Lupidon H respectively, although nothing is known as yet regarding the working principle of this therapy, which bases on immunological reflections. It was, therefore, examined by the complement fixing and neutralizing antibodies against herpes simplex virus of 11 patients and 3 control individuals if repeated injections of this vaccine cause alterations of the specific antibody titer. Skin tests were made in addition with 9 patient prior to and 8 weeks after therapy at the earliest. During the entire eamination period of at least 8 weeks up to 9 months the complement fixing as well as the neutralizing antibody titer remained practically constant and in none of the examined cases the skin tests caused any reaction. Since it is, however, known, e.g. from the examinations by Lehner, that seropositive individuals reacta to herpes virus with symptoms of an allergy of the delayed type, is must particularly be considered in furture examinations on the working mechanism of Lupidon that the Lupidon solutions for intracutaneous testing must be of considerably higher concentration. The questition of the site of action of Lupidon at the interferon mechanism remains pending in addition.