Anaphylatoxins, in particular C3a and C5a, have various biological activities which suggest a role as mediators of inflammatory reactions: they cause contraction of smooth muscle, histamine release, increase in capillary permeability, adhesion of leukocytes to vascular endothelium, leukocyte chemotaxis, and aggregation of platelets and leukocytes. Most of these effects are supported by the cooperation of other mediators, in particular arachidonic acid derivatives which may be produced by anaphylatoxin-stimulated cells, e.g. leukocytes or endothelium. In vivo effects of the complement peptides depend very much on the site of their generation: intravascular release in the general circulation leads to adverse symptoms such as adult respiratory distress syndrome and shock lung, mainly due to leukocyte activation, aggregation and their accumulation in lung vessels. Intravascular release may be induced by certain drugs, and by contact of blood with the surfaces of bypass or dialysis apparatus. Induction of local inflammatory and defense reactions requires release of anaphylatoxins in tissue spaces. Tissue fluid differs quantitatively from blood plasma in its concentration of complement components. This raises some problems of how efficient concentrations of C3a and C5a can be attained at the site of a lesion to generate a chemotactic gradient capable of attracting blood leukocytes.