Thirty-two different full-length cDNA clones for human killer cell inhibitory receptors (KIR) have been identified. They all belong to the immunoglobulin superfamily and encode mature proteins with one, two or three extracellular Ig domains. The inhibitory receptors have nearly identical transmembrane domains and cytoplasmic domains ranging in length from 76 to 95 amino-acid residues. The activating receptors have a characteristic transmembrane domain with a charged lysine residue and a short cytoplasmic tail without the protein tyrosine phosphatase binding motif I/VXYXXL present in the inhibitory receptors. Sequence analysis demonstrates that three clusters correspond to the inhibitory receptors of 58 kDa, while two clusters encode activating receptors of 50 kDa. Two other clusters correspond to the inhibitory receptors of 70 kDa and one cluster encodes genes with sequence homology to one of the two p70 clusters but contains the transmembrane and cytoplasmic domains characteristic of activating receptors. The data are consistent with a genomic organization of the KIR genetic region containing at least three KIR genes encoding receptors for each of the gene products of the HLA class I loci. Alternative mRNA splicing could be responsible for generation of activating or inhibitory receptors with the same extracellular domains. Separate genes encoding receptors with opposite function is, however, an equally likely possibility.