Transfused leukocytes from adult rats do not transmigrate efficiently in neonatal rats. The ordinate indicates the percentage increase (see Materials and Methods) in cpm of radiolabeled adult leukocytes that transmigrated into the peritoneum of neonatal and adult rats. The bars represent the mean ± SEM of values from 4 rats in each group. The difference in the percentage increase in peritoneal leukocytes between the newborn and the adult is statistically significant (P = .05).