Adhesive interactions involving sickle erythrocytes are critically important in the pathophysiology of vaso-occlusive crisis, hemolytic anemia, and other clinical manifestations of sickle cell disease. These interactions appear to be mediated by the abnormal expression and/or function of adhesion molecules on the surface of sickle erythrocytes. Recent studies in model membranes and intact biological systems have begun to elucidate the biochemical and biophysical properties of adhesion molecules that are necessary for stable adhesion; these properties include adhesion molecule expression, size, lateral mobility, surface density, surface distribution, and affinity for the molecule's cognate ligand. Although a number of molecular interactions involved in sickle lerythrocyte adhesion to vascular endothelial cells and T lymphocytes have been identified, the properties of these molecular interactions that are important for stable adhesion remain to be characterized. We have developed a unique set of biophysical and imaging techniques to study, at the level of individual adhesion molecules, the molecular interactions involved in cell-cell adhesion. The methods include fluorescence photobleaching recovery, polarized fluorescence depletion, single particle tracking, laser optical tweezers, glass-supported planar bilayer membranes, fluorescence resonance energy transfer, and dynamic in vitro and in vivo (intravital) adhesion assays. Here we propose to apply these methods to the study of (1) membrane protein iand lipid dynamics in sickle erythrocytes, (2) adhesive interactions between sickle erythrocytes and activated vascular endotheliai cells, and (3) adhesive interactions between sickle erythrocytes and activated T lymphocytes. We shall use these methods to study adhesive interactions involving the adhesion molecules VLA-4 (alpha4beta1integrin), CD36, and CD2 on sickle erythrocytes, VCAM-1 and alpha-v, beta3 integrin on activated vascular endothelial cells, CD58 on activated T lymphocytes, and the adhesive plasma protein thrombospondin. Results from these studies are expected to lead to a quantitative understanding of important molecular and cellular events in the pathophysiology of sickle cell disease, and, ideally, to point the way to targeted therapies that interrupt the most critical aspects of these molecular and cellular events.