The changes in MCV at various times after renal transplantation were studied in a group of patients with functioning cadaveric renal grafts who had received azathioprine from the time of transplantation and in a constant dose from 2-8 weeks later. A post-transplantation increase in the MCV was seen in the majority of the cases studied and in these cases the MCV did not reach a maximum value until as long as 11-40 months after transplantation. In most cases the MCV then feel although frequently not to within the normal range and in some cases it showed further fluctuations. These data suggest gradual alterations over a period of years in the various cytokinetic changes which serve to maintain a reasonably high output of red cells from the marrow despite the cytotoxic effects of azathioprine.