Alloreactivity of human NK cells was tested on a subset of the 4AOHW cell panel. A total of 37 cells were typed with NK clones reactive with the NK-1,2,3, and 5 allospecificities. No cell was susceptible to lysis by both by anti-NK-1 and anti-NK-2 clones in accordance with the notion of a biallelic system, where the susceptibility toward lysis is a recessive trait. HLA homozygous cells were lysed either by the anti-NK-1 or anti-NK-2 clones while HLA heterozygous cells in some cases were not lysed by either clone. Negativity for the NK-1 specificity corresponded to the presence of asparagine and lysine at positions 77 and 80, respectively, in the second exon of HLA-C (alleles Cw2, 4, 5, and 6), while negativity for the NK-2 group corresponded to the presence of serine and asparagine, respectively, at these two positions (alleles Cw1, 3, 7, and 8). Too few cells were typed with clones reactive with the NK-3 and NK-5 specificities to enable an analysis of correlation between these specificities and HLA allelus.