Two subpopulations of lymphocytes in the peripheral blood of young, conventionally reared Tamworth pigs had the following cell-surface markers analogous to those of human lymphocytes; (1) T cells--receptor(s) for ovine and caprine erythrocytes, permitting spontaneous formation of rosettes and the presence of the thymus-derived antigen; and (2) B cells--membrane-associated immunoglobulin and receptor(s) for complement. The T cells could be identified by either the erythrocyte rosettes or the cytotoxicity test, and the B cells could be identified by either the erythrocyte-antibody-complement rosette test or the immunofluorescence test for surface immunoglobulin. Because of simplicity, the cytotoxicity and immunofluorescence tests are recommended. The cell losses in ficoll-diatrizoate gradient centrifugation were studied with 9 blood samples. Of the total T and B cells, 71.7% were in fraction 1, and 28.3% were in erythrocyte fraction 2. Recovery rates of total leukocytes varied widely in relation to leukocyte numbers present in the original blood; the range was between 52.6 and 93.8%. Moreover, evidence did not indicate that either T or B cells were selectively sedimented in fraction 2. Analysis of the lymphocyte-rich fractions (fraction 1) from 37 blood samples by ficoll-diatrizoate gradient centrifugation indicated the following: (1) Approximately 2.7 X 10(6) T cells and 0.9 X 10(6) B cells could be obtained in the fraction from 1 ml of peripheral blood; (2) T and B cell populations were independently distributed; hence, no definite ratio between T and B cells applied to an individual animal; (3) approximately 20% of the lymphocytes did not have markers as either T or B cells; and (4) an average of 26% cells were peroxidase-positive leukocytes, chiefly monocytes.