“…With the exception of the GRA curve after ni trogen mustard (see above), the shapes of the dose response curves for the 3 assays are as expected for three different cell populations proliferat ing at different rates [5]. These results, therefore, favour the existence of different populations of stem cells, and would be consistent with the con siderable amount of evidence from other types of investigation [1,2,7,8,17,18] which have suggested the existence of different populations of re populating cells committed to separate lines of haemopoietic differentia tion. The present studies with proliferation-dependent cytotoxic agents then lead to the conclusion that the erythroid repopulating cells prolifer ate rather more quickly, and the granulocytic repopulating cells, some what unexpectedly, more slowly than the CFU.…”