1. In mice rendered lymphocytopenic by X-irradiation or hydrocortisone acetate, pertussis vaccine evoked both lymphocytosis and polymorphonuclear leukocytosis.
2. When mice with lymphocytosis induced by pertussis vaccine were X-irradiated, prompt and extensive destruction of circulating as well as tissue small lymphocytes occurred. The devitalized circulating cells were cleared from the blood primarily by the Kupffer's cells of the hepatic sinusoids.
3. Hydrocortisone acetate administered to mice with lymphocytosis did not cause acute lymphopenia nor was there any evidence of destruction of circulating small lymphocytes. However, destruction of these cells within lymphoid tissues was apparent. These observations suggested that adrenal cortical hormones are not "lymphocytolytic" with respect to circulating lymphocytes.