In vivo cytotoxic responses to NIMA-like alloantigens develop during the neonatal period. (A) Neonates were injected with allogeneic adult spleen cells or PBLs. 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. (B) (BALB/c × FVB) F1 neonates were injected with semiallogeneic adult (C57BL/6 × FVB) F1 spleen cells, as described for Figure 2. Seven days later, neonates were injected with a 1:1 mixture of Far Red–labeled (C57BL/6 × FVB) F1 and (BALB/c × FVB) F1 cells, as described in “In vivo cytotoxicity assays.” The percentage of allospecific cytotoxicity in the spleen (SP) and lymph nodes (LNs) was determined as described for Figure 3. Data are pooled from 3 independent experiments; n = 31 neonates.