Vaccinia vims-mediated gene transfer is a powerful technique to test the function of individual receptors in natural killer (NK) cells. By transient expression of genes encoding NK receptors, one can obtain high levels of protein synthesis, while maintaining NK cell functions. The vaccinia virus expression system is useful for the analysis of wild-type and mutant forms of NK receptors and for the study of structure-function relationships as they relate to receptor-ligand interactions and signaling events.