CTL activity in vitro. CTL activity was determined by⁵¹Cr release assay. Equal numbers of splenic T cells from a 6-day primary MLR (B6 anti-B6D2F1) were used as effector cells. p815 cells (H-2^d) and EL4 cells (H-2^b) were labeled with ⁵¹Cr and used as targets. After 4 hours of coincubation with effector cells, ⁵¹Cr in the supernatants was determined by a γ-scintillation counter. One of three representative experiments is presented.