The occurrence and properties of leukocytosis and lymphocytosis-stimulating material in the supernatant fluids of Bordetella pertussis cultures.

1. Leukocytosis- and lymphocytosis-stimulating activity was present in fluid cultures of B. pertussis. The activity was found primarily in the culture supernatant fluid. 2. The sequential changes in the leukocyte response were similar to those previously observed following injection of intact bacteria into mice. 3. Activity was destroyed by heat and was diminished, but not abolished, by prolonged treatment with proteolytic enzymes. 4. A water-insoluble fraction of the culture supernatant fluid was isolated which contained virtually all of the activity. The specific activity was more than 100-fold greater than that of the intact bacteria, and injection of microgram quantities produced a response. 5. The distribution of histamine-sensitizing factor followed that of leukocytosis-stimulating activity. In contrast, mouse protective antigen was localized to the bacterial pellet.