Thymidine suicide and hydroxyurea kill ratios accurately reflect the proliferative status of stem cells (CFU-S).

We have previously found that on average 30% of hematopoietic stem cells (CFU-S) are in the S phase, which is at least three times the value published by others. Therefore, it seemed desirable to investigate the reliability of the methods used to measure the percentage of CFU-S in S phase. Various modifications of the [3H]thymidine suicide were tested and it could be demonstrated that results were not affected by them. Furthermore, results obtained with the [3H]thymidine suicide were compared to those obtained with methods utilizing hydroxyurea to kill CFU-S in S phase. The [3H]thymidine- and hydroxyurea-based methods gave parallel results. The parallel behavior validated all of these methods as reliable indicators of the turnover of CFU-S and presumably other stem cell populations.