The leukocytosis- and lymphocytosis-promoting factor (LPF) of Bordetella pertussis has been isolated to near homogeneity by physical, chemical, and electron microscopical criteria. LPF contains 14.5% nitrogen and is lipid and carbohydrate free. It is apparently composed of four polypeptide subunits. LPF caused leukocytosis and lymphocytosis in "nude" as well as in normal mice. In addition, purified LPF also induced histamine sensitization and hypoglycemia and refractoriness to the hyperglycemic effect of epinephrine. A monospecific LPF antiserum blocked these reactions as well as leukocytosis and lymphocytosis. LPF is clearly distinct from the hemagglutinating pili of B. pertussis.