Natural killer (NK) cells are controlled by receptors specific for polymorphic determinants of class I molecules of the major histocompatibility complex (MHC). The contrasting properties of NK and cytotoxic T cell (CTL) class I receptors provide complementarity in the cytolytic lymphocyte response to viruses, tumours and transplants. Whereas human NK cell class I receptors consist of immunoglobulin domains, their mouse counterparts resemble C-type lectins. This difference may reflect the receptors' diverse and rapidly evolving class I ligands.