Strath®/Bio-Strath® is a plasmolyzed herbal yeast product with malt, honey and orange syrup. Its ability to support the immune system of the human being is well known and was tested in several studies. Equi-Strath®, the adapted product for horses was tested in this study. The aim of the study was to analyze the effects of Equi-Strath® on skin condition in the horse. The study included twenty healthy Franches-Montagnes stallions which were divided into two groups. The study group (SG, n = 10) received 0.06 ml/kg bodyweight of Equi-Strath®, the control group (CG, n = 10) the same amount of placebo, daily. The supplement and the placebo were given from week 1 until week 10 of the trial, orally. Skin biopsies were taken three times, once at the beginning of the study (week 0), once at half time (week 5) and once at the end of the study (week 10), followed by histological analysis including leucocytes count twice in five vision fields of each slide. No alterations of hair, adnexal structures or skin thickness were found. However, the number of perivascular lymphocytes turned out to be increased at the end of the study in 9 out of 10 animals of the study group, whereas number of lymphocytes in biopsies of the control group where increased only in 4 out of 10 animals. The increase of lymphocytes in the study group suggests an activation of the immune system due to the application of Equi-Strath®.