The anaphylatoxins exhibit three primary functions in vivo, i.e. they induce granulocyte sequestration and adherence, promote spasmogenesis or smooth muscle contraction and exert immunoregulatory influences. C5a receptors have been demonstrated on human neutrophils, human monocytes, rodent macrophages from both the lungs and peritoneal cavity, two mouse macrophage cell lines (RAW 264 and J774.A1) and guinea pig platelets. To date, only guinea pig platelet has been shown to possess C3a receptors. In depth studies with the rat mast cell have demonstrated that both direct and indirect activation mechanisms may be operative in the mast cell; however, neither mechanism appears to depend on receptor-mediated stimulation of these cells.