Multinucleated red cell precursors (A) displaying grossly abnormal features relative to normal erythroblasts (B) that reflect the bone marrow toxicity in a patient receiving azathioprine who had self-administered allopurinol for an attack of gout. Allopurinol interferes with degradation of azathioprine to cause a 3- to 4-fold increase in its cytotoxic capacity. This can result in severe and potentially fatal pancytopenia. Patients taking azathioprine must be alerted to this danger of co-medication with allopurinol.