Development of the trisomy 19 mouse thymus was studied by (1) evaluation of cell and nuclear counts and volume distributions and (2) examination of the in vivo cell cycle characteristics using flow cytometry. Direct preparations of thymuses from affected animals and their normal littermates at 17, 18, 19 gestational days, and at term were used. Cell numbers in aliquots from the trisomic thymus suspensions were always significantly less than those in the normal animals. There were no differences in cellular or nuclear volumes between the two groups. However, the populations of cells in the trisomic suspensions showed a lag in the normally occurring transition from a larger to a smaller size. DNA distribution histograms from these suspensions were generated using a TPS-1 Cell Sorter, and percentages of cells in different phases of the cell cycle were estimated using computer analysis. The resulting graphs and numerical data from the trisomic mice, when compared with the normals, showed consistent increases in the relative numbers of cells in the S and G2 + M phases of the cell cycle. Because growth retardation is a major feature of trisomy 19, as well as in other murine trisomies, these results suggest that cell cycle alterations may, in part, contribute to the reduced cell number, delayed development, and smaller size of the trisomic animals.