The relationship of rodent granulated metrial gland (GMG) cells to the natural killer (NK) cell lineage is reviewed. The antigenic profile of GMG cells provides insufficient evidence to indicate that these cells form part of the NK cell lineage. No good evidence is found that GMG cells are cytotoxic to the NK cytotoxicity target, Yac-1. In Beige mice, which have a defect in the lytic pathway of NK cells, GMG cells were found to kill labyrinthine cytotrophoblast. It is concluded that GMG cells are not a type of NK cell but a novel member of the leucocyte population.