To investigate whether allergen-specific IgE production is influenced by environmental and genetic factors, IgE levels against 2 mould extracts (Alternaria alternata [Alt a] and Aspergillus fumigatus [Asp f]) and against recombinant (r) rAlt a 1, rAsp f 7 and rAsp f 8 were determined by ELISA in sera from 448 Lipizzan horses living in 6 studfarms. Statistical evaluation showed a significant effect of studfarm-specific environment on IgE levels against the different allergens, but genetic factors also influenced allergen-specific IgE production: an heritability of 0.33 was found for IgE levels against the 2 mould extracts and of 0.21 for rAsp f 8-specific IgE. Heritability estimates for rAlt a 1- and rAsp f 7-specific IgE were negligible. Investigations for a possible association between Major Histocompatibility Complex (MHC) class I antigens and specific IgE levels were carried out. The most consistent significant association was found between the equine leucocyte antigen (ELA) A8 and undetectable IgE titres against rAsp f 7 and rAsp f 8. Significant ELA associations were also demonstrated between ELA A1 and higher specific IgE levels, between ELA A14 and lower IgE levels against the mould extracts and in one studfarm between ELA Be27 and lower Aspergillus-specific IgE levels.