An electron microscopic study of human natural killer (NK) cells showed abundant cytoplasm and relatively large mitochondria. They also had various kinds of granules in their cytoplasm. The mode of binding to the tumor cell (K 562) was tight, and villi protruded from NK cells to the target cells. Some target cells were already lysed after incubation for 5 minutes at 4°C.

This work was supported in parts by Grants-in-Aid for Scientific Research (Project Nos. 544050 and 56770468) from the Ministry of Education, Science and Culture of Japan.