Two genes, alpha and beta, encode murine low-affinity receptors for the Fc portion of IgG (Fc gamma RII). The amino acid sequences deduced from the nucleotidic sequences of alpha and beta cDNA are highly homologous in extracellular domains. As a consequence, the protein product of the alpha Fc gamma R gene has not yet been distinguished from that of the beta Fc gamma R gene. alpha and beta cDNA, however, show no homology in sequences coding for intracellular portions. We therefore raised antibodies against a synthetic peptide corresponding to the 26 intracytoplasmic amino acids of the expected product of the murine alpha Fc gamma R gene. F(ab')2 fragments of alpha-specific antibodies thus obtained stained specifically membrane proteins which were present in cells containing alpha transcripts but not in cells containing beta transcripts only; they bound molecules carrying the 2.4G2 epitope, characteristic of the extracellular domains of murine Fc gamma RII; they immunoprecipitated material which migrated as heterogeneous glycosylated proteins of 45-55 kDa when native and, when deglycosylated, as a single polypeptide with an apparent molecular mass of 29 kDa, which is compatible with the calculated molecular mass of the protein expected to be translated from alpha Fc gamma R transcripts. These criteria identify a product of the murine alpha Fc gamma R gene as a subtype of Fc gamma RII which can be designated Fc gamma RIIa.