Intracerebral injection of vaccine into the mouse induced swelling of the brain. The swelling reached the maximum in the intensity by day 1 and persisted for several days. A method for quantitative determination of the brain-swelling activity of the vaccine was developed. A positive regression coefficient was found only between the brain-swelling and the lymphocytosis-promoting activities. Such activity was no longer shown with the vaccine heat-treated for 30 min at 80 C, but it was restored upon addition of the lymphocytosis-promoting factor (LPF) that caused no brain swelling by itself. The activity, therefore, was ascribed to cooperation of LPF and a certain heat-stable component other than endotoxin contained by pertussis vaccine.