At least three distinct DNA polymerases, named alpha, beta, and gamma, have been isolated from normal mammalian cells. The function of these enzymes in regard to DNA replication and repair remains unclear. Stimulation of blood lymphocytes with the plant mitogen phytohemagglutinin (PHA), is known to increase total DNA polymerase activity. In this study, we measured the change of each of these activities as lymphocytes intered a mitotic cycle. Aliquots of a pool of normal human blood lymphocytes were incubated with PHA for 0, 24, 48, and 72 hr, respectively, and the various DNA polymerase activities quantitated at each point. No significant DNA polymerase activity was detected in unstimulated cells. Low levels of polymerase beta were found at 24 hr. The average DNA content per cell doubled between 24 and 48 hr, and during this interval all three DNA polymerases increased to easily detectable levels. By far the greatest fractional increase in activity of all three polymerases was seen between 48 and 72 hr, after the average doubling of cellular DNA. In summary, these blood lymphocytes lack significant levels of DNA polymerases; stimulation with PHA induces all three of the major DNA polymerase species. In both these respects, these cells differ from other proliferating mammalian cell systems. The possible significance of this difference is discussed.