The average doubling times of HL60 cells grown in the presence of 0,5,10 and 100 microM 3'-azido-3'-deoxythymidine (AZT) for 72 h were, respectively, 51.1, 65.7, 69.0 and 76.3 h. This drug-concentration-dependent prolongation of the cell-doubling time was associated with a progressive increase in modal cell volume. The technique of combined Feulgen microspectrophotometry and 3H-thymidine autoradiography revealed that the cell cycle distribution of HL60 cells cultured in the presence of 5 and 100 microM AZT for 48 h was abnormal, with an increased percentage of cells in the S phase and a decreased percentage in the G1phase. From the cell doubling times and the cell cycle distribution data, and making a number of assumptions, upper limit estimates for the duration of the S phase in cultures containing 0,5 and 100 microM AZT were calculated to be 26.2, 35.8 and 49.6 h, respectively. The data indicate that concentrations of AZT achieved in the plasma of patients receiving this drug (i.e. 5 microM) cause a substantial prolongation of both the cell cycle time and the duration of the S phase of HL60 cells. It therefore seems likely that some of the toxic effects of AZT seen in vivo, including impairment of bone marrow function, are at least partly related to AZT-induced disturbances of DNA synthesis and proliferation in human cells.