The main purpose of this experiment was to test the effect of Insol® Dermatophyton (ID) by employing a placebo controlled blind study, testing 51 horses suffering from summer eczema. The animals were vaccinated three times every two weeks in February of the year, in which the experiment took place, using ID (n=27) or placebo (n=24). Furthermore, the testing encompassed showing whether immunomodulatory effects could be detected when having used ID. For this purpose, each time the horses were tested –5 times in total– blood serum samples were taken to evaluate specific cytokines (IL-1, IL-4, IL-10, IFNγ, TNFα). Changes of the clinical findings (alteration of skin, itching) were assessed by a score especially developed for this purpose, before, during and after the insect period of the respective testing year. Following the generally well-known predilection sites, 12 body regions were specified, each of which was analysed for five symptoms (hair breakage, alopecia, bloody/purulent skin alteration, skin thickening, dandruff) and rated using points between 0 (not apparent) and 5 (severe). By adding and evaluating these points, for each animal and each time-point, the clinical intensity of the summer eczema could be quantified. After having used ID in a therapeutic or prophylactic placebo controlled blind study, it could not be established that there was a significantly positive effect on the clinical findings of skin affected by summer eczema. However, a positive trend could be observed in that the ID vaccinated animals in summer had a lower score than the ones which had been treated with a placebo. The score, nevertheless, was not statistically significant. This trend, however, could be confirmed by an owner-survey comparing the year of treatment with the year before. On this occasion the owners of the verum group evaluated the clinical appearance of the symptoms as significantly improved in the year of treatment compared with the year before. This effect could not be seen in the placebo group. Furthermore, the “young eczema” horses treated with ID (with symptoms of 2 years maximum) had a respectively lower score than the “old eczema horses” each time they were examined. Concerning the cytokines, an immunomodulatory effect resulting from the therapy could be verified. For instance, the concentrations of IL-10, TNFα, and IFNγ in the verum group rose considerably and what is also statistically significant, they rose after the vaccination, too. This is to be interpreted as an immunomodulatory shift from Th2 (allergy) to Th1 (tolerance). However, this effect could only be shown in-vitro. Further studies will have to evaluate, if the immunomodulatory effect can be intensified by changing of the ID formula, hence allowing statistical evidence in-vivo.