Human erythrocytes are equipped with a diversity of receptors and effectors that mediate well-characterized signal transduction pathways in nonerythroid cells. Some of these erythrocyte components may be vestiges of signaling pathways critical to the functions of the erythrocyte's precursors but no longer needed in the mature erythrocyte. Other signaling elements, however, are likely involved in enabling the erythrocyte to detect and respond to the needs of other hematopoietic and endothelial cells with which it comes in contact.