Small round cells which migrated from explant cultures of rat metrial gland were identified as granulated metrial gland (GMG) cells. They contained large amounts of glycoprotein and displayed the leucocyte common antigen. Other cells which migrated from the explants were probably derived from the fibroblast-like stromal cells of the metrial gland. The asialo-GM1 antigen was found on rat GMG cells in culture and in cryostat sections of rat metrial gland. The rat GMG cells in culture exhibited locomotion and, when co-cultured with placental cells, made numerous contacts with the placental cells. A small number of these contacts (less than 1 per cent) were followed rapidly by the death of the placental cell. Mouse GMG cells which had migrated from explant cultures of mouse metrial gland were also co-cultured with rat placental cells. The migratory activity of the mouse GMG cells also involved numerous contacts being made with rat placental cells and a small number (less than 1 per cent) of these contacts were cytotoxic for the rat placental cells. The observations support previous suggestions that GMG cells are a type of killer cell. The cytotoxic activity of rat and mouse GMG cells against co-cultured rat placental cells is discussed in relation to the nature of the target molecule involved.