The isolation of mucosal mast cells and globule leucocytes from the small intestine of sheep immunized with Trichostrongylus colubriformis is described. Sheep mast cell protease was released from these cells in a dose-dependent fashion after incubation with soluble protein from T. colubriformis larvae. Release also occurred with other T. colubriformis antigens whereas non-parasite antigens at comparable protein concentrations evinced only a minimal response. Mucosal mast cells prepared from worm-free sheep also produced a similar minimal response. This is the first report describing the release of sheep mast cell protease from isolated sheep intestinal mucosal mast cells after addition of specific parasite antigens.