Mobilized peripheral blood progenitors (CD34+ cells) have been shown to be either in the G0 or G1 phase of the cell cycle. In this study, it is shown that they are small cells with low protein content suggestive of G0. Support for this is provided by showing that the principal E2F complex consists of hypophosphorylated p130, E2F-4, and DP-1. The E2F-4 is more highly phosphorylated than in quiescent T cells. In response to cytokines in vitro, the CD34+ cells start to enter G1 within 8 hours and enter S-phase at about 48 hours. As cells enter G1, E2F-4 is dephosphorylated to several hypophosphorylated forms and three new DNA-binding complexes appear, including one containing E2F-4, DP-1, and p107. We suggest that mobilized CD34+ cells may be maintained in G0 by p130, E2F-4, and DP-1 and the coordinate dephosphorylation of E2F-4 and hyperphosphorylation of p130 may be central to the initiation of proliferation.