Certain membrane molecules are enzymatically cleaved from the cell surface and then released into the extracellular medium in the form of soluble fragments. This process, commonly initiated by cell stimulation, may regulate the surface expression of such molecules, and may also be responsible for the production of their soluble forms in vivo. Here, Vladimír Bazil provides an overview of the molecules that are cleaved from cells, focusing particularly on leukocyte receptors. In addition, he discusses the mechanisms and putative enzymes involved in this process, as well as the potential physiological significance of such events.