Ribonucleotide reductase catalyzes the first unique, rate-limiting step in DNA synthesis; both its large (M1) and small (M2) subunits are necessary for activity. While direct studies of M2 expression have previously shown a tight correlation with S phase, the kinetic features of M1-expressing cells have remained ill defined. Therefore we have, using immunofluorescence flow cytometry, analyzed changes in whole cell M1 levels and DNA content during various cell cycle and differentiation events. In asynchronous cultures M1 levels are sustained throughout the cell cycle, including G1 phase when M2 levels and ribonucleotide reductase catalytic activity are known to be very low. In contrast M1 is virtually absent from quiescent lymphocytes but is expressed following mitogen stimulation, shortly before S phase cells appear. M1 declines to low levels in "plateau phase" cultures, the major reduction occurring in cells with 2n (G0/G1) DNA content. HL-60 promyelocytic leukemia cells, induced into either myeloid or monocyte-macrophage differentiation, show a similar marked decrease in M1 levels concomitant with the cessation of cell division. We conclude that the M1 subunit of ribonucleotide reductase is constitutively expressed by cycling cells. It is acquired during stimulated transition from G0 to G1 and is lost during exit to G0 or terminal differentiation. This pattern of expression suggests that determination of cellular M1 content may be useful in distinguishing proliferating (including G1) and quiescent (including G0) cells in vivo.