We have characterized the rat NK receptors NKR-P1A and -P1B. A cDNA library was constructed from the rat NK cell line, RNK-16. Using the pMX retroviral cloning system, the library was expressed in the human NK cell line, YTSeco, and cells staining with the anti-rat mAb 10/78 identified, FACS sorted and cloned. Two genes, corresponding to rat NK receptors NKR-P1A and -P1B, were identified. YTSeco clones expressing either NKR-P1A or -P1B were functionally tested using (51)Cr-release redirected lysis assays and calcium flux experiments. This demonstrated that NKR-P1A functions as an activation receptor, as previously shown, and that NKR-P1B functions as an inhibitory receptor, as predicted by the presence of an immunoreceptor tyrosine-based inhibition motif. Although annotated as NKR-P1A specific, we found that mAb 10/78 stained YTSeco clones expressing NKR-P1A or -P1B equally well, as did the mAb 3.2.3 used for the original cloning of rat NKR-P1A.