Whole blood from two male individuals was X-irradiated using a linear accelerator at 200 cGy/min to give a total exposure of 300 cGy. Lymphocytes were cultured using standard techniques with the addition of 3 micrograms/ml cytochalasin B at 26 h to produce binucleation through the inhibition of cytokinesis for the scoring of micronuclei after the first nuclear division. Replicate cultures from each individual were harvested at 48, 68, 70, 72, 74, 76, 92, 94, 96, 98 and 100 h postinitiation using a cytocentrifuge. Slides were stained with acridine orange, and binucleated cells were scored for the presence of micronuclei. There were no statistically significant differences in the frequency of micronucleated binucleates between replicate cultures, between individuals, or among cultures harvested from 48 to 100 h postinitiation. This indicates that the phytohaemagglutinin-stimulated lymphocytes are a relatively homogeneous population of cells with respect to X-radiation-induced chromosome damage. In addition, these data show that for determining the frequency of micronuclei in lymphocytes irradiated in G0, the harvest time (up to at least 100 h postinitiation) is not critical as long as analysis is confined to the first mitosis after irradiation (i.e. the binucleated cells).