Rabbit antirat B-cell serum (ALS_B) reacted selectively with the surfaces of lymphocytes in the lymphoid follicles of lymph node cortex and in the follicles and marginal zones of splenic white pulp, but not with the surfaces of germinal center cells or plasma cells. An identical pattern of fluorescent staining was obtained with rabbit antirat Ig serum. It was shown by blocking, absorption, and immunoprecipitation studies that ALS_B was composed in large part of antibodies to rat Ig, but that it contained antibodies to other B-cell antigens as well.

Rabbit antirat T-cell serum (ALS_T) reacted selectively with the surfaces of lymphocytes in the paracortex of lymph node and in the periarteriolar sheath of spleen, and with thymocytes. ALS_T did not display anti-Ig activity.

ALS_T reacted with approximately 100% thymocytes and with 90% thoracic duct, 80% lymph node, 60% blood, 50% spleen, and 10% bone marrow lymphocytes in suspensions of cells from these sources. ALS_B reacted with the remainder of the lymphocytes in the suspensions, except for bone marrow in which only 59% of lymphocytes had detectable B- or T-cell surface antigens.

The population of T lymphocytes in rat bone marrow was depleted by drainage of lymphocytes from a thoracic duct fistula, thereby establishing their membership in the pool of recirculating T cells. Approximately 14% of lymphocytes issuing from the thoracic duct of TxBM donors reacted with ALS_T. The presence in these animals of a small number of T cells, calculated to be approximately 2% of the normal value, may account for the limited capacity of TxBM rats to respond to antigens that induce a cell-mediated immune response.