Effect of hydroxyurea on donor stem cell engraftment in hosts irradiated with 100 cGy. To selectively kill cells transiting S phase, donor male BALB/c mice received an injection of 900 mg/kg hydroxyurea 3 hours before death. Mice injected with PBS constituted the control donor group. The two cell populations were infused into separate groups of irradiated (100 cGy) female hosts and the percentage of male cells in female host marrow was assessed 8 weeks after cellular infusion. Actively cycling cells will be killed by hydroxyurea and noncycling cells will be spared. The present results are from two experiments with 5 mice per experiment and show no significant differences, indicating that the engrafting cells were quiescent.