SummaryIn order to investigate the properties of the trisomic cells in patients with Down's syndrome, comparisons of the cell cycle, durations of component phases and chromosomal sensitivities were performed both in trisomic and normal lymphocyte cultures, using aH-thymidine autoradiography. In the present experiments, the presence of the two cell sub-populations was found in trisomic cells as well as in normal cells.The durations of the G2 + M and S phases were similar in both subpopulations, 3.9 and 8.1 hr for trisomic cells and 4.6 and 7.9 hr for normal cells, respectively. For the longer cell cycle sub-population, the duration of the total cell cycle and the G 1 phase were 23.1 and 11.1 hr for the trisomic cells and 20.4 and 7.9 hr for the normal controls. For the shorter cell cycle subpopulation, those were 13.2 and 1.3 hr for the trisomic cells and 12.7 and 0.8 hr for the normal controls, respectively.Chromosomal sensitivities to incorporated 3H-TdR were similar in two sub-populations within a group, but significantly higher chromosome aberration yields were observed in the trisomic cells.