The Xenopus laevis oocyte has been widely utilized for cloning and functional expression of G-protein coupled receptors (GPCR). This system was used for the functional expression and characterization of the recently identified human C3a receptor. Complementary RNA from the human C3a receptor was transcribed in vitro and microinjected into Xenopus oocytes for functional characterization. A positive response to a synthetic C3a peptide agonist and to C3a, but not to platelet activating factor or fMetLeuPhe was detected. In addition, a response of approximately one third the amplitude obtained with C3a was obtained with rC5a. Conversely, oocytes co-injected with the C5a receptor and total RNA isolated from U937 cells responded to C5a as well as to C3a and the C3a synthetic peptide. A functional response with the anaphylatoxin C3a receptor in oocytes was dependent on co-injection of a pertussis toxin sensitive complementary human factor which could be supplied by co-injection of total RNA isolated from U937 cells. Oocytes expressing the anaphylatoxin C3a and C5a receptors responded to both agonists, in each case the response to the cognate ligand was substantially more robust than the response elicited by the other anaphylatoxin.