The product of the retinoblastoma susceptibility gene RB1 (Rb) is likely to function as an inhibitor of cell growth. Previous studies have suggested that certain growth-suppressing effects of Rb are exerted in G0/G1 phase and that phosphorylation can inactivate these functions. We tested this hypothesis by examining the expression and state of phosphorylation of Rb in several lineages of primary hematopoietic cells that spontaneously arrest in G0 phase. Resting lymphocytes were found to express only unphosphorylated Rb, but phosphorylation of Rb occurred as the cells entered S phase in response to mitogens. In contrast, although monocytes and granulocytes also expressed high levels of unphosphorylated Rb, these terminally differentiated cells did not phosphorylate Rb, nor could they exit from G1 phase in response to growth factors. Thus, Rb phosphorylation appears linked to the ability of a cell to synthesize DNA. In T and B lymphocytes, Rb protein increased 8-fold after stimulation, while RB1 RNA levels increased 2- to 4-fold. Nuclear run-on assays and measurement of RB1 RNA half-life in T cells suggested that the increased RNA abundance was, at least in part, due to increased RNA stability. By contrast, Rb protein levels did not increase in either monocytes or granulocytes after stimulation, although RB1 RNA levels did increase in monocytes. Thus, there are lineage-specific differences in both the regulation of Rb phosphorylation and RB1 gene expression in lymphoid and myeloid cells.