['Cyathostomins are the principal pathogenic nematode of equidae worldwide. In other species mast cell (MC) proteinases, in particular chymases, appear to have protective roles. Knowledge of the equine intestinal immune response to cyathostomins is limited.', 'To investigate MC numbers and proteinase expression in equine cyathostomin-infected large intestine.', 'MC populations in the large intestine are positively associated with cyathostomin burden and predominantly express chymase.', "The caecal cyathostomin burden of naturally infected horses (n = 25) was determined by luminal counts and pepsin digest (mural count). MC were identified and enumerated in caecal tissue using toluidine blue (TB). Immunofluorescent labelling with polyclonal rabbit antibodies was used to demonstrate expression of equine tryptase and the chymase equine mast cell proteinase-1 (eqMCP-1) in Carnoy's fixed caecal sections.", 'Significant positive linear relationships were found between TB-stained mucosal and submucosal MC counts and total cyathostomin burden (P<0.001, r² >36%), and both luminal (P<0.010, r² >25%) and mural (P<0.001, r² >36%) larval counts. Similar relationships were found with mucosal and submucosal chymase and tryptase-labelled MC counts (total: P<0.004, r² >29%; luminal: P<0.004, r² >30%; and mural: P<0.030, r² >19%). With all three MC labels, mean MC counts were higher in the submucosa compared to the mucosa (P<0.001). All caecal MC appeared to express chymase, with a small number of MC expressing both tryptase and chymase.'] Is large intestinal mast cell count and proteinase expression associated with larval burden in cyathostomin-infected horses?