The treatment of human myeloid leukemia cell lines with phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), is associated with loss of proliferative capacity and induction of monocytic differentiation. The present results demonstrate that treatment of asynchronous human U-937 leukemia cells with 10 nM TPA is also associated with oligonucleosomal DNA cleavage. This pattern of DNA fragmentation, which is observed in programmed cell death, was detectable in populations of TPA-treated cells that had entered a nonproliferative G0/G1 phase. Similar findings were obtained after TPA treatment of a synchronous population of G1 cells. These cells progressed through S and G2/M phases before undergoing internucleosomal DNA cleavage during G0/G1 arrest. These G0/G1 cells displayed characteristics of monocytic differentiation, including down-regulation of c-myc expression and induction of c-fms transcripts. DNA fragmentation was also studied in cells treated with 5 nM TPA for 48 h and then monitored in drug-free long-term culture. Endonucleolytic cleavage was similarly observed in the differentiated G0/G1 population. However, longer periods of culture were associated with a decrease in DNA fragmentation to undetectable levels. This effect was followed by retrodifferentiation and reentry of cells into cycle. Taken together, these findings demonstrate that internucleosomal DNA fragmentation occurs during induction of monocytic differentiation, and that both of these events are detectable in G0/G1 cells.