The relationship of alterations in blood flow with changes in cell distribution has been studied in an inflammatory site and its draining lymph node during the induction of an immune reaction with oxazolone in mice. The cells which move to the site of inflammation are predominantly lymphoblasts and their increased localization in the inflamed ear is significantly correlated with increased regional blood flow to the inflamed tissue. The existence of this correlation is not antigen dependent although there is a relative increment of lymphoblasts which are specifically primed to the inflammatory agent. The localization of nonblastic (small) 51Cr-labeled lymphocytes on the other hand is substantial only in lymphoid tissue and during the induction of an immune reaction after oxazolone application, the increase in localization of these cells in the draining lymph node is positively correlated with increased blood flow to the node. Furthermore, the probability of finding 51Cr labeled lymphocytes in a particular lymph node is related to the regional blood flow which that node receives.