1. Intravenous injection into mice of phase I Bordetella pertussis vaccine resulted in a striking hyperleucocytosis with a predominating lymphocytosis. Intraperitoneal inoculation was less effective, and subcutaneous administration was inactive.
2. Active immunization prevented the hyperleucocytosis; passive immunization was less effective.
3. Reticuloendothelial blockage reduced the effect of the vaccine.
4. Extirpation of the spleen or thymus did not alter the leucocyte response.
5. Histologic studies suggested that the increase in circulating lymphocytes resulted from release of cells from lymphoid organs, including the thymus.