Polymorphism in the alleles of the killer cell immunoglobulin-like receptor 2DL1 and 2DS1 genes has been investigated by the development of polymerase chain reaction–sequence-specific oligonucleotide probing systems. The methods have been applied to 77 Northern Irish families, establishing allele frequencies from the unrelated parents. Additionally, cell line DNA from individuals and CEPH families of the 13th International Histocompatibility Workshop panel were investigated. Eight of the reported KIR2DL1 alleles and only the KIR2DS1*002 allele were identified in the groups studied. Two individuals were positive for three alleles of KIR2DL1, and a putative variant of KIR2DL1*001 was observed. Results also indicated an inherited KIR2DL1/2DS1 splice variant present in a haplotype with several core loci absent, in two families.