Recently we recognized that pertussis toxin (PT) increased vascular permeability, as does cholera toxin. This reaction can be observed as a blueing reaction by intradermal inoculation of PT into the adult rabbit skin followed by, about 7h later, an intravenous administration of pontamine sky blue solution. The permeability factor (PF) of PT represented exact linear relationship with the dose of PT. This reaction was shown to be more sensitive than leukocytosis-promoting and histamine-sensitizing phenomena due to PT. Furthermore, above three reactions were neutralized similarly and specifically by anti-PT antibody. These results strongly suggest that the PF activity is one of the biological functions of PT, and the skin test is simple, sensitive and accurate and applicable for estimating the PT activity.