Trauma of daily white cell counts on mice leads to a lymphocytosis in strains C57Bl, LAF1 and Rf but not in the high-leukemia strains AkR and C58. Thymectomy prevents lymphocytosis response to trauma. Injection of cell-free thymus extracts into thymectomized mice restores ability to develop lymphocytosis. Injections of lymph node or spleen extracts are ineffective. Lymphocytosis response is not dependent on adrenal function.