Murine natural resistance to hemopoietic allografts, including F1 hybrid resistance to parental grafts, is a manifestation of natural killer (NK) cell's alloreactivity. The genetic basis of natural resistance has long been a puzzle, because the resistance is specific and is controlled by genes linked to the H-2, the mouse major histocompatibility complex (MHC). Recent studies by us and others suggest that H-2 class I molecules are the major determinants that control the specificity of this resistance. In fact, our study suggests that the prototypical hybrid resistance of B6D2F1(H-2b/d) mice to parental B6 (H-2b) graft is mostly, although not completely, mediated by NK cells expressing Ly-49A, a receptor that receives an inhibitory signal from Dd and Dk. Thus, the absence of particular class I molecules appears to define the specificity of this resistance, rather than positive recognition of putative target determinants controlled by the Hh-1 locus. This view is also supported by another study of ours in which Dd-loss variant clones of cell lines derived from parental DBA/2 (H-2d) and syngeneic B6D2F1 mice were found to express a phenotype indistinguishable from that of parental B6. The results indicate that susceptibility to killing by B6D2F1 NK cells does not require genes specific for H-2b. This favors the concept that lack of Dd expression is the necessary and sufficient condition for the susceptible phenotype hitherto known as Hh-1b. The nature of the receptors expressed by Ly-49A subset of B6-specific B6D2F1 effectors and the structural requirements of the Dd molecule that prevents the recognition by Ly-49A+ and Ly-49A- subsets of these effectors, will be examined as one of the objectives of the proposed studies. Our preliminary results also suggest that similar resistance of B6 mice against BALB/c (H- 2d) cells may be controlled by a different mechanism, possibly involving an H-2-independent determinant and a determinant which is positively contributed by the Dd molecule. Analysis of this resistance is therefore the second main objective of the proposed studies. Finally, we propose to analyze murine NK cell alloreactivity and its relationship to the spectrum of Ly-49 family members expressed at the level of single clones. This is based on our preliminary results, in which multiple Ly-49 transcripts were detected by RT-PCR in single clones. These studies, therefore, are designed to elucidate the molecular basis of the specific murine natural resistance against hemopoietic stem cells and neoplasm, and will hopefully suggest a means of reducing bone marrow allograft failure in man.