Various phospholipases, enzymes which cleave fatty acid groups from phospholipids, are activated following chemotactic factor receptor ligation. Three major mammalian phospholipases are present in leukocytes and are designated, phospho-lipase A₂, phospholipase C and phospholipase D. The majority of the chemotactic factors whose receptors are coupled to G proteins stimulate a rapid increase in the level of intracellular calcium, an event that is dependent on activation of a phosphoinositide-specific phospholipase C. All of the chemotactic factors tested to date are able to induce arachidonic acid release in target leukocytes, although recent data suggest that leukocytes must be primed with granulocyte macrophage-colony stimulating factor for this to occur. The availability of antiphosphotyrosine antibodies has enabled the characterization of the role of tyrosine phosphorylation in chemotactic factor receptor signal transduction pathways. With few exceptions, leukocyte chemotactic factor receptors are members of the superfamily of G protein-coupled receptors.