The leukocytosis activity of C5a was studied in a rabbit model. Blood samples were drawn for cell counts through a catheter placed in an artery of one rabbit ear after injection of either porcine or human C5a into a vein in the opposite ear. These studies indicate the potential of C5a to mobilize bone marrow neutrophils following transient C5a-induced neutropenia, based on counts of nonsegmented neutrophils. The numbers of circulating neutrophils can be selectively elevated by 300% to 400%, within 1 to 2 hours, in rabbits given only microgram quantities (1 to 5 x 10(-9) mol/l) of C5a or C5adesArg. These quantities of C5a are equivalent to 1% to 2% of complement C5 activation. The time-course of the induced neutrophilia is characteristic of C5a, increasing rapidly in the first 10 to 20 minutes after injection and attaining a maximum level at 2 to 5 hours, then decreasing slowly to normal levels over the next 4 to 6 hours. After boiling at 100 degrees C for 10 minutes, C5a (C5ades Arg) lost its leukocytosis activity, indicating that the cellular effect was not caused by endotoxin. Other known leukocytosis factors, such as epinephrine, dexamethasone, lipopolysaccharide, and the prostanoid 15(S)-15-methyl PGF2 alpha, produced a distinctly different profile of leukocyte mobilization than that of C5a. The C5a-induced neutrophilia was not inhibited by pretreating these animals with indomethacin, suggesting that it is not a prostanoid-induced effect. One hypothesis is that no secondary cellular mediator system is involved in C5a-mediated leukocytosis, but rather than C5a alone is responsible for a rapid mobilization of neutrophils from bone marrow pools, and perhaps marginated pools, following neutropenia. Circulating neutrophils are activated by C5a and thereby become deformed and adherent, leading to a neutropenia, sequestration, and depletion of cells. After the neutropenic event an immediate neutrophilic response is required for replacement of this particular cell population and to re-establish homeostasis. Therefore the role of C5a may be just as important as other known leukocytosis factors (fragments from C3, for example) in promoting complement-dependent neutrophil mobilization in response to tissue injury, infections, or extracorporeal blood treatments.