Significantly more peripheral blood mononuclear cells (PBM) from 39 patients with lymphoma (p less than 0.001), myeloma (p less than 0.001), or leukemia (p less than 0.001-0.025) bind transferrin (Trf) than those of 23 normals. That Trf was bound to a receptor on these cells was shown by the inhibition of these cells bound to 125I-Trf with unlabeled Trf. The paucity of Trf receptors on PBM from normals and their presence on diseased cells prompt a suggestion that Trf receptors may offer a useful approach to the diagnosis and possible treatment of some blood dyscrasias.