We have recently described a novel paired immunoglobulin-like receptor (PIR) gene family, PIR-A and PIR-B, that is coordinately expressed by murine B and myeloid cells. To examine the conservation of the PIR gene family, we performed Southern blot analysis of human genomic DNA using a PIR cDNA probe. Several discrete DNA fragments cross-hybridized with the mouse probe under high stringency and were isolated from a genomic library. Sequence analysis of genomic clones revealed six exons with 45 to 65% homology to the mouse PIR gene. The genomic DNA fragment was used to probe a human leukocyte cDNA library. The positive clones obtained belong to a recently described family of human immunoglobulin (Ig)-like receptors (ILT/MIR/LIR/HM43) that contain four Ig domains and are expressed on B and myeloid lineage cells. Like the murine PIR family, these new human Ig-like receptors are of two types, an activation type with a short cytoplasmic tail and a charged transmembrane region and an inhibitory type with four candidate immunoreceptor tyrosme-based inhibitory motifs. The human activation type receptor appears to be encoded by a very limited number of genes, in contrast to the mouse PIR-A multigene family. Conversely, the single mouse PIR-B gene is represented by several different human counterparts. Comparative DNA blot analysis of human genomic DNA with the human Ig-like receptor and the murine PIR probes shows the same banding pattern, suggesting that there are no other PIR homologues.