Thirteen, 7 months old thoroughbred horses with no clinical signs of insect bite hypersensitivity, commonly exposed to culicoides bites were studied. Allergic individuals were determined by a positive intradermal skin test, performed with culicoides allergen. Animals with negative skin test served as a control. In horses serum levels of IgG were examined. From horses with positive skin test results, skin biopsies were examined by means of light and electron microscopy. Positive reaction against culicoides allergens confirmed that IgG even in absence of IgE can mediate allergic response in <1 year old horses. Ultrastructural skin studies showed marked activation within mast cells population in allergic horses even though they exhibited no clinical symptoms of summer eczema. There was marked domination of piece meal type of degranulation observed which indicated persistent inflammatory process within skin of asymptomatic animals. Moreover, skin mast cells of allergic horses whose IgG serum levels were relatively high besides piece meal often manifested anaphylactic and mixed type of degranulation.