Unlike the human FcγRII and FcγRIII families, which exhibit considerable diversity at both the nucleic acid and protein levels, the human FcγRI family has only a single recognized product expressed as a 70-kD cell surface receptor with high affinity for monomeric IgG (hFcγRIa1). Using both polymerase chain reaction-based amplification and Northern hybridization, we document multiple interferon-γ-inducible hFcγRI RNA transcripts in human mononuclear cells and neutrophils. The sequences of two of these FcγRI related transcripts indicate that they are alternatively spliced products of a second FcγRI family gene, termed FcγRIB. The cDNA derived from the larger of these transcripts, termed hFcγRIb1, encodes a surface molecule that is not recognized by FcγRI specific monoclonal antibodies when transfected into COS-7 cells. Unlike the interferon-γ-inducible hFcγRIA gene product, hFcγRIb1 does not bind monomeric IgG with high affinity. However, both hFcγRIa1 and hFcγRIb1 do bind aggregated human IgG. Previously unrecognized diversity within the hFcγRI family includes an interferon-γ-inducible, putative low affinity Fcγ receptor that may play an important role in the mechanism by which Fcγ receptors participate in the humoral immune response.