Erythroid toxicity of azathioprin. Macrocytosis and selective marrow hypoplasis.

In a population of 70 stable renal allograft recipients receiving azathioprine, macrocytosis was present in a majority and its severity was related to azathioprine dosage. There were associated megaloblastoid marrow changes which could not be related to serum folate or vitamin B12 concentrations. Two cases of selective red cell hypoplasia in renal allograft recipients are described; in each patient, azathioprine was incriminated and the effect dose-related. These observations indicate that the erythroid effects of azathioprine may precede, and be independent of, its effects on the myeloid series.