The relative cytotoxic effector cell capabilities and the proportions of cells bearing various surface markers in human tonsil and peripheral blood mononuclear cells has been studied. The peripheral blood contained a substantial proportion of monocytes (22 +/- 2.9%) compared to tonsil cell suspensions (2.5 +/- 0.3%). The percentages of T lymphocytes was significantly higher in the blood than in the tonsil (P is less than 0.01); however, the percentages of cells forming rosettes with 7S EA were not significantly different in each group (P greater than 0.5). Mitogen-induced cellular cytotoxicity by blood and tonsil mononuclear cells against Chang cells was proportional to the percentages of T lymphocytes in these cell suspensions, and both antibody-dependent and mitogen-induced cellular cytoxicity against sheep red blood cells was proportional to the percentages of monocytes in these suspensions. Tonsil mononuclear cell suspensions were incapable of mediating antibody-dependent cellular cytotoxicity against Chang cells, whereas blood mononuclear cells functioned normally. These findings are in contrast to the findings of similar percentages of Fc receptor-positive lymphocytes in blood and tonsil mononuclear cell suspensions. Previous studies have shown that the effector cells against antibody-coated Chang cells are Fc receptor-positive lymphocytes. These studies show that in the case of cytotoxicity mediated by an Fc receptor-bearing lymphoid cell, there may be a clear discrepancy between the relative proportions of Fc-bearing lymphoid cells in different organs and the relative levels of cytotoxicity.