In humans, three distinct but closely related classes of receptors that bind the Fc portion of IgG (FcgammaRI, II and III) have been identified. FcgammaRI can bind monomeric IgG with high affinity and has a unique third extracellular domain (EC3). Three very similar genes have been characterized for FcgammaRI (A, B, C). Although the sequences are remarkably similar, a number of coding-region differences discriminate between the genes and amongst their transcripts. Six distinct FcgammaRI transcripts were analysed. Three transcripts, one from each gene, contain all six exons. Only the gene A transcript appears to encode a bona fide high affinity receptor, a three Ig-domain membrane spanning receptor that can bind monomeric IgG. Stop codons in the EC3 domains of the gene B and gene C isoforms would be predicted to generate secreted receptors. Three transcripts are alternatively spliced isoforms, one from gene A and two from gene B. One gene B transcript encodes a two Ig-domain transmembrane receptor which has structural characteristics of a low affinity FcgammaR.