Demonstration of a time-dependent loss in competitive repopulating ability of W⁴¹/W⁴¹ FL CRUs by comparison with their +/+ FL counterparts. +/+ FL cells (5 × 10⁴) were injected into irradiated recipients either alone or together with increasing numbers of W⁴¹/W⁴¹ FL cells in the ratios shown. Percentage of repopulation refers to the proportion of circulating WBCs derived from the injected +/+ cells at different times after transplantation. Results from two experiments have been pooled to calculate the mean ± SEM values shown (n = 6 to 14 animals per group). The differences between the percentage of repopulation values by +/+ FL cells after 5, 8, and 16 weeks in mice that received only +/+ FL cells are not significant (P < .05). The differences between the percentage of repopulation values by +/+ FL cells after 8 and 16 weeks in mice that received W⁴¹/W⁴¹ FL cells as well as +/+ FL cells are significantly different from the values obtained in the corresponding groups after only 5 weeks (P ≤ .002).