We discovered a phenomenon in which the blood flow in vein microcirculation markedly decreases in response to hen-egg white lysozyme (HEL)-sensitization without any change in blood pressure. Using this blood flow decrease as a guide, we developed an in vivo assay method to search for substances, which can prevent allergies. Antagonists of histamine, serotonin and platelet activating factor (PAF) did not affect the blood flow decrease in response to HEL-sensitization. On the other hand, cyclooxygenase (COX)-1, COX-2, thromboxane (TX) A(2), endothelin-1 (ET-1), prostacyclin (PGI(2)) and granulocytic elastase (GE) as well as nitric oxide (NO) from inducible NO synthase (iNOS) were involved in the blood flow decrease. Thus, these substances might injure vascular endothelial cells, and cause a decrease in blood flow in vein microcirculation. Our method can be used to search for preventive agents against allergies involving NO, COX-1, 2 and PGI(2). This is the first report to applying to an assay method the specific blood flow decrease to occur in the promotion stage of allergy.