Antisetum against purified cytoplasmic granules from rat LCL tumor cells. and protein A-gold immunoelectron microscopy were used to study the secretory events in lysis of YAC-1 tumor cells by rat LCL tumor cells or by isolated LGL from normal rats. After 30 min incubation of effector and target cells together

gold-labeled cytoplasmic granules were often seen concentrated in the area of the LGL adjacent to the bound YAC-1 target. Within 60 min

the granules were observed to move to the cell border near the conjugated site. At this point

the granules were fused with the cell membrane. and subsequently released the gold-labeled contents into the junction between the LGL and the target cell. Gold particles could be seen at the E-T interface

on the LGL surface

or sometimes on the target cell surface. These data provide direct evidence for the hypothesis that under conditions of active cytotoxicity

natural killer cells secrete their cytoplasmic granule contents leading to the deposition of granule material on the target cell surface and the eventual lysis of the cell.