From 1981 until 1987 we investigated the more detailed circumstances regarding a prophylactic vaccination in altogether 37 stud farms with a history of virus abortion. In 23 cases, in which Prevaccinol and/or Resequin were used, it was found that the following of vaccination schedule and necessary immunization programmes respectively, had considerable imperfections. In seven cases prophylactic vaccinations were not carried out or corresponding questionnaires were not answered. The fact that in the present data no case of virus abortion was observed, when the mare was vaccinated according to the instructions within a regular vaccinated population, certainly speaks for the vaccination. Of course the inquiries made have also shown that in practice it is very difficult to meet all the requirements fixed in the instructions for immunization. But with regard to the general problems at control of herpes virus infections and at avoidance of any discrimination of the vaccination, these requirements may not be loosened. Those who are concerned must be aware of the fact that every variation of the vaccination instructions is linked to an increased risk of illness and that vaccination cannot substitute a sound management.