Although the restriction point (R-point) was proposed in animal cells several decades ago, its existence in normal cells is still controversial, because, in most studies, long-term cultured cell lines rather than primary normal cells were used. Furthermore, cell synchronization was generally applied, resulting in growth imbalance between DNA synthesis and protein expression in cells. Finally, R-point was originally proposed as a unique arrest point that may be in G(0) phase; however, generally believed R-point locates within G(1) phase. Thus, up to now, there is no solid experimental evidence that supports the existence of R-point in asynchronous primary normal cells. In this study, we used freshly purified peripheral human blood lymphocytes, as asynchronous primary normal cells, to confirm the existence of restriction point in G(1) not G(0) phase. Our findings may help uncover the mystery of the deregulation of cell cycle progression in malignant tumors. (c) 2013 International Society for Advancement of Cytometry