T and B lymphocytes were detected in human adenoid, tonsil, appendix and Peyer's patches by adherence to sheep erythrocytes (E) and human erythrocytes sensitized with antibody and complement (HEAC). The percentages of T lymphocytes in adenoid and tonsil cell suspensions averaged 30.9 +/- 3.4 (S.D.) and 35.8 +/- 6.4 (S.D.). The percentages of B lymphocytes in the same tissues were 42.5 +/- 11.3 (S.D.) and 40.1 +/- 34 (S.D.) respectively. In adenoid and tonsil tissue sections, B lymphocytes were found in the follicles and T lymphocytes were detected around the follicular areas. The predominant cell population in Peyer's patches and appendix sections was constituted by B lymphocytes.