Cycling characteristics of human lymphocytes in vitro

The cycling characteristics of human peripheral blood lymphocytes in phytohemagglutinin stimulated cultures were examined by means of a new technique employing radioautographic methods which assay the fraction of cells whose nuclei contain DNA polymerase utilizing exogenous and/or endogenous DNA primer template capability. The fraction of cells labeled by these techniques increases 5 - 11 hr prior to the onset of DNA synthesis. Estimates of cell cycle time, G ₁ time, and fraction of cycling cells are made for the first 4 days in culture. Evidence is presented to support the hypothesis that daughter cells of the first divisions may be retired from cycle by virtue of loss of primer template availability, rather than by loss of DNA polymerase.