The mouse anaphylatoxin C3a receptor (mC3aR) gene was isolated using a human C3aR cDNA probe. The genomic fragment contains an open reading frame of 1431 base pairs that encodes a peptide of 477 amino acids. A cDNA with identical sequence was subsequently obtained from the mouse pre-B cell line 70Z/3 by reverse transcriptase polymerase chain reaction based on sequence of the mC3aR gene. Northern blot analysis suggested expression of the mC3aR in lung and heart, and to a lesser extent, in brain, liver, muscle, kidney, and testis. The deduced amino acid sequence of the mouse C3aR is 65% identical to that of the human C3aR. Like the human receptor, mouse C3aR contains a predicted large extracellular loop of approximately 165 amino acids (residues 161-325) between the fourth and fifth transmembrane domains. This loop, however, is the least conserved structure (45% identical sequence) of all the extracellular and intracellular domains between the mouse and human C3aRs. The mouse gene product bound 125I-labeled human C3a with a Kd of 2.54 nM when expressed in the stably transfected rat basophilic leukemic cell line RBL-2H3. Bound C3a could be effectively displaced by excess quantities of unlabeled C3a, but not by C4a or C5a. C3a induced dose-dependent calcium mobilization in the transfected cells, which could be blocked by pertussis toxin treatment. These results confirm that the cloned gene encodes a functional C3aR capable of coupling to a pertussis toxin-sensitive G protein. The sequence divergence of the large extracellular loop does not appear to affect C3a binding and transmembrane signaling.