We describe three situations in which a large fraction of circulating red blood cells attach tightly and specifically to fibronectin: (0) rabbits made anemic by repeated bleeding, (ii) patients with hemolytic anemia and functional asplenia and splenectomized normal humans, and (iii) splenectomized mice. Upon induction of anemia in rabbits, the proportion of circulating red blood cells capable of specifically attaching to fibronectin-coated plastic increased in parallel with the number of reticulocytes. Fibronectin-adherent red cells were barely detectable when the rabbit had recovered from the anemia. Attachment of reticulocytes to fibronectin was specific; cells did not attach to dishes coated with albumin, laminin, or collagen. None of these proteins promoted the attachment of normal erythrocytes. About 75% of the erythrocytes from splenectomized mice (but not control mice) also attached specifically to fibronectin 40 days after surgery. The effect of splenectomy was incomplete and transient; adherent cells were not detectable 8 weeks after splenectomy. As judged by labeling studies with [35S]methionine, newly emergent reticulocytes preferentially attached to fibronectin. We suggest that about half of the reticulocytes in erythropoietically unstressed mice lose their ability to attach to fibronectin, possibly due to loss of fibronectin-adhesive components, during passage through the spleen. The others lose their ability to interact with fibronectin before release, in the bone marrow, or in some extrasplenic site.Erythrocyte production in adult mammals begins in the bone marrow; nucleated erythroid stem cells undergo proliferation and terminal differentiation into nonnucleated reticulocytes, the immediate precursor of mature erythrocytes. Before the final stages of maturation, reticulocytes are released from the bone marrow into the circulation. Reticulocyte maturation is completed within 1-3 days; during this period remnants of mitochondria, Golgi complex, and polyribosomes are eliminated. Reticulocyte maturation also involves the remodeling of the plasma membrane, resulting in loss of the ability to exhibit endocytosis (1-3).Undifferentiated murine erythroleukemia (MEL) cells growing in culture adhere tightly to fibronectin, and erythroid differentiation is accompanied by the loss of adhesion fibronectin (4). Fibronectin is a glycoprotein associated with extracellular matrices of many tissues; it is tightly bound to the surface of many cells, and is also found in the blood plasma (5, 6). Fibronectin is a major component of the interstitial matrix in the bone marrow (7). We speculated that the bone marrow precursors of erythrocytes interact with fibronectin and that loss of fibronectin adhesion is associated with release of the cell into the circulation (4). But the function of such interactions during erythroid differentiation remains to be investigated.We show here that human, rabbit, and mouse reticulocytes specifically attach to fibronectin and that the remodeling of the reticulocyte plasma membran...