Cultured human lymphocytes treated with phytohemagglutinin (PHA) had higher rates of RNA and protein synthesis, as judged by incorporation of the labelled precursors 3H-uridine and 14C-leucine, than did control cultures without PHA. These cultures were prepared from cells of the same donor, as were a third set of cultures which were equilibrated with 2 per cent halothane. The increased rates of RNA and protein synthesis six and 16 hours after PHA addition were inhibited by the halothane, modestly at six hours and strikingly at 16 hours. These experiments provide further evidence that halothane prevents recruitment of resting cells into the active cycle of cell division.