Mast-cell-deficient W/W(v) mice were sensitized by oral administration of 0.1 and 1.0 mg ovalbumin (OVA) by gavage every day for 9 weeks, and active systemic anaphylaxis (ASA) was induced by intraperitoneal injection of OVA. The production of OVA-specific IgE and IgG1 by oral immunization of the W/W(v) mice was high, and the production of IL-4 by splenocytes re-stimulated with OVA in vitro was increased. In contrast, production of OVA-specific IgG2a and IgG2b was low, and production of IFN-gamma by splenocytes after re-stimulation with OVA in vitro was rather decreased. These findings suggest that Th2-dominant helper T-cell activation had occurred. No increase in serum histamine level was observed following ASA induction. However, the plasma platelet-activating factor (PAF) levels of the mice sensitized with 0.1 and 1.0 mg OVA by gavage increased significantly. The increases in plasma PAF correlated well with the ASA-associated decreases in body temperature, suggesting that PAF plays an important role in ASA in W/W(v) mice. Taken together the above findings indicate that W/W(v) mice are a good model not only for studying induction of food allergy but also for examining the role of PAF in food-induced hypersensitivity.