Platelet-activating factor (PAF) reacts with a specific seven transmembrane, G protein-linked receptor with two promoter splice forms showing tissue-specific regulation and for which a knockout mouse has recently been created. The receptor is widely expressed on hematopoietic cells, endothelial cells, keratino-cytes, and cells of the central nervous system. Л huge number of receptor antagonists have been synthesized and many are being explored in vitro, in animal models, and in a variety of clinical trials. In addition, the complexity of the PAF story is increased by the observation that in the majority of cells that synthesize it, most of the PAF remains within the cell, leading to speculation that it plays an intracellular role, perhaps even on an intracellular receptor, in addition to its role as extracellular communication molecule.