SUMMARYCellular and humoral responses of mice were quantified after allografting across an H-2 histocompatibility barrier (H-2b-H-2a). Both i.p. tumor grafts and skin grafts were used. The in vitro cytotoxic activity of sensitized lymphocytes correlated with in vivo homograft rejection in both normal animals and mice with impaired immunity produced with antilymphocyte serum. Lymphoid organs differed in their response to the various allografts. Cellular immune activity was highest in the regional lymph nodes during the 1st week following the challenge by skin allograft. On the other hand, immune activity was greatest in the spleen and mesenteric node following the i.p. challenge with tumor cells. No one lymphoid organ reflects the quantitative changes in the cellular immune responses of all the lymphoid organs during the homograft reactions. These studies emphasized the need for assays which are capable of measuring the cellular and humoral immune responses, separately and apart from the intact animal, to provide information on the nature of the homograft reaction.