Human blood lymphocyte fractionation with special attention to their cytotoxic potential

In studies concerning the natural cytotoxic activity of human blood lymphocytes we have found that: 1. T cells, which rosette with sheep erythrocytes can be separated by centrifuging on Ficoll the lymphocyte and erythrocyte mixture without previously pelleting and incubating in the cold. 2. Lysis of marker erythrocytes with ammonium chloride impairs the cytotoxic activity of lymphocytes. Incubation at 37°C for 12 hr prior to the test restores the activity to some extent. 3. A high proportion of the so called 'null' fraction i.e. cells remaining in the interface after removal of B cells (passage on nylon column) and sedimentation of E rosettes, sediments as rosettes with sheep EAC' indicating that these cells carry low density E and C₃ receptors. Rosetting with SRBC or ox EAC' gave significantly lower values. On a per cell basis the 'null' fraction was the most efficient one in natural cytotoxicity. Depletion of cells with low affinity E or E and C₃ receptors left highly active subfractions.