Cytolytic activity of NK1.1⁺ cells derived from immature NK cell colonies. Fetal thymocytes from C57Bl/6-Ly5.1/C57Bl/6-Ly-5.2 F1 mice were cultured under the conditions described in the legend for Fig 3. After 8 days of fetal thymus organ culture, Ly5.1⁺NK1.1⁺ cells were prepared by FACS sorting from the cells in the fetal thymus lobe culture, cultured for an additional 7 days with IL-2 (500 ng/mL), and assayed for cytolytic activity as described in Materials and Methods. As a control, the NK1.1⁻ cells from the immature NK cell colonies were assayed for cytolytic activity. (○), NK1.1⁺ cells from the fetal thymus culture; (•), cells from the immature NK cell colonies.