In the plasma membrane of animal cells, many membrane-spanning proteins exhibit lower lateral mobilities than glycosylphosphatidylinositol (GPI)-linked proteins. To determine if the GPI linkage was a major determinant of the high lateral mobility of these proteins, we measured the lateral diffusion of chimeric membrane proteins composed of normally transmembrane proteins that were converted to GPI-linked proteins, or GPI-linked proteins that were converted to membrane-spanning proteins. These studies indicate that GPI linkage contributes only marginally (approximately twofold) to the higher mobility of several GPI-linked proteins. The major determinant of the high mobility of these proteins resides instead in the extracellular domain. We propose that lack of interaction of the extracellular domain of this protein class with other cell surface components allows diffusion that is constrained only by the diffusion of the membrane anchor. In contrast, cell surface interactions of the ectodomain of membrane-spanning proteins exemplified by the vesicular stomatitis virus G glycoprotein reduces their lateral diffusion coefficients by nearly 10-fold with respect to many GPI-linked proteins.