The various pooled populations of cells from upper, middle, and lower areas of the gradient were characterized using autoradiographic, TCA-precipitable ³H]thymidine incorporation, and Fuelgen microspectrophotometric methods. The upper range of the gradient contains cells in the G₁ cell cycle phase; the lower range, cells in the G₂ phase; cells found in the middle of the gradient belong largely to the S phase of the cell cycle.

These gradient-separated cell pools contained relatively little contamination with cells from other phases of the cell cycle and, when explanted from the gradient into fresh growth media, showed growth patterns characteristic of synchronized cell populations. This system of cell separation provides a useful tool for investigating the relationship of the cell cycle to surface membrane and metabolic characteristics in human lymphoid cell culture systems.