A number of cell culture techniques are now available to evaluate clonogenic hematopoietic progenitors as well as cell populations that represent an interactive network of cells with regulatory capabilities. Although clonogenic cells in allografts do not appear to be correlated with the time to recovery, they seem to predict their time to engraftment in autografts. However, these cell populations are likely to be only surrogate measurements because a reduction by various purging techniques does not seem to alter the ability of autografts to reestablishing hematopoiesis. Currently, it is not known whether or not these differences may relate to differences in the preparative regimens, graft-versus-host disease prophylaxis, or other differences in the management, such as administration of high concentrations of various antiviral agents. It is also now possible to measure growth factor production following the transplant procedure and to correlate the availability of endogenous factors and recovery after a transplant. Under normal conditions, the activity levels appear to return to normal values within 20-30 days, whereas in patients with late engraftment one may observe prolonged elevation of growth factors. If confirmed, this would suggest that in bone marrow transplants a delayed return is more related to reduced progenitor cells rather than a deficiency of growth factors.