The number of colony-forming units thymus are reduced in aged D2 but not in B6 mice. Shown is the frequency of CFUt per 10⁵ cells injected in B6 (left panel, mean and SD of 3 experiments) and D2 mice (right panel, 1 experiment using 20 hosts). Animals were injected with mixtures of young (y) and aged (a) BM with an excess of host-type BM. CFUt frequencies were calculated from the number of mice that had thymic donor-type cells derived from either the aged or the young donors at 4 weeks. For details, see “Colony-forming unit thymus assay.”