Abstract
A concept has been suggested that the role of immunogen is to stimulate resting cells to enter a phase of the cell cycle in which the synthesis of immunoglobulin is obligatory. This process conceivably involves the initial union of cells with immunogen followed by a subsequent transition from resting to proliferating cell. Several aspects of an in vitro cellular transition have been investigated using cultured WIL~2~ lymphocytes which are shown to enter the G~1~ phase of the cell cycle upon release from rest. This transition is associated with phenotypic changes in the cells manifested by differences in density of individual cells and the amount and profile of polyribosomes. An increase in the rate of synthesis of total protein and specific immunoglobulin polypeptides accompanies the G~0~ to G~1~ transition. Agents useful in bacterial and other mammalian cell systems to probe translational versus transcriptional control mechanisms are active in these lymphocytes. This cellular model appears to offer unique opportunities to approach regulatory problems in cell biology because large numbers of synchronized cells are obtainable in which specific messengerβRNAs and their corresponding polypeptides can be isolated in relatively pure form.