The mechanism by which Bordetella pertussis organisms and their products induce lymphocytosis in mice was analyzed in terms of the localization of syngeneic Cr-51-labeled lymph node cells. Labeled lymphoid cells incubated in vitro with the supernatant of B. pertussis cultures and then injected intravenously into normal recipients, or labeled cells injected into pertussis-treated recipients were unable to "home" to lymphoid organs but persisted for long periods in the blood. In animals "equipped" with a population of Cr-51-labeled lymphoid cells, administration of B. pertussis organisms or culture supernatant effected a shift of radioactivity from lymph nodes and spleen into the peripheral blood, coincident with the lymphocytosis. In in vitro experiments it was found that the active principle could bind to both erythrocytes and lymphocytes and could spontaneously elute from these cells onto labeled lymphocytes which were then unable to home efficiently. The data suggest that Bordetella pertussis -induced lymphocytosis involves a reversible attachment of the pertussis factor onto the surfaces of lymphocytes which prevents their recirculation to lymphoid organs. Recirculating lymphocytes are presumably affected as they emerge from lymphoid organs to enter the blood.

The effects of heterologous rabbit anti-mouse lymphocyte antiserum on the morphology of lymphoid and other tissues was investigated in CBA mice. The lymphoid tissues exhibited characteristic changes specific for ALS treatment, which were an invariable accompaniment to its immunosuppressive effects. These consisted of peripheral lymphopenia occurring at some time during a course of ALS treatment and persistent depletion of small lymphocytes in lymph node paracortical areas and splenic follicular periarteriolar zones. The thymic histology was generally well preserved. It is suggested that the relevant lesions reflect a rapid depletion of the pool of recirculating lymphocytes, possibly by a primary cytotoxic effect exerted on cells peripheral to lymphoid tissue. Other histologic features attendant to the administration of ALS were accounted for as consequences of immunization of ALS recipients to rabbit serum constituents or by the deleterious effects of antibodies directed against tissues other than lymphoid cells.