NK1. 1 alloantigen expression can be used to define NK cells in certain mouse strains, such as B6 (NKR-P1C) and SJL (NKR-P1B). However, BALB/c NK cells do not react with the anti-NK1. 1 mAb, PK136. To investigate the NK1. 1− phenotype of BALB/c NK cells, we have undertaken NK1. 1 epitope mapping and genomic analysis of the BALB/c Nkrp1 region. Bacterial artificial chromosome library analysis reveals that, unlike the Ly49 region, the Nkrp1-Ocil/Clr region displays limited genetic divergence between B6 and BALB/c mice. In fact, significant divergence is confined to the Nkrp1b and Nkrp1c genes. Strikingly, the B6 Nkrp1d gene appears to represent a divergent allele of the Nkrp1b gene in BALB/c mice and other strains. Importantly, BALB/c NK cells express abundant and functional Nkrp1 transcripts, and the BALB/c NKR-P1B receptor functionally binds Ocil/Clr-b ligand. However, the BALB/c NKR-P1B/C sequences differ from those of the known NK1. 1 alloantigens, and epitope mapping demonstrates that directed mutation of a single amino acid in the NKR-P1B BALB protein confers NK1. 1 reactivity. Thus, PK136 mAb recognizes, in part, a distal C-terminal epitope present in NKR-P1B Sw/SJL and NKR-P1C B6, but absent in NKR-P1A/D/F B6 and NKR-P1B/C BALB. Allelic divergence of the Nkrp1b/c gene products and limited divergence of the BALB/c Nkrp1-Ocil/Clr region explain a longstanding confusion regarding the strain-specific NK1. 1 alloantigen reactivity of mouse NK cells.