In vivo exposure to NIMA-like allogeneic cells induces striking phenotypic changes on neonatal spleen cells. Neonatal and adult BALB/c (H-2^d) mice were injected with C57BL/6 (H-2^b) adult spleen cells, as described in “Preparation and injection of donor cells.” Seven days later, spleen cells were harvested and analyzed by flow cytometry. Spleens were pooled from 2 to 3 mice per group. (A) Host-specific class I MHC expression. The depicted histogram is representative of 12 independent experiments. (B) Host-specific class II MHC expression. The histogram shown is representative of 8 independent experiments. (C) Expression of class II MHC on F4/80⁺ spleen cells. The histograms are representative of 6 independent experiments.