IFNγ production by donor cells contributes to the development of neonatal allospecific cytotoxic responses. Adult C57BL/6 (A,B), CD40L^−/− spleen cells (A), or IFNγ^−/− spleen cells (B) were transplanted into BALB/c neonates, as described for Figure 2. Seven days later, the percentage of allospecific cytotoxicity in the spleen (SP) and lymph nodes (LNs) was determined, as described for Figure 3. Data are pooled from 2 to 4 independent experiments. **P = .02; *P = .04, compared with wild-type donor cells by t test.